USMLE Microbiology Part 1

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  • 1/25 Questions

    A 4-month-old infant presents with failure to thrive, progressive muscular weakness, and poor head control. On questioning, the mother states that she typically feeds the baby soy-based formula sweetened with honey. Which of the following organisms is most likely to be responsible for the child's presentation?

    • Clostridium botulinum
    • Clostridium difficile
    • Clostridium perfringens
    • Clostridium tetani
    • Corynebacterium diphtheriae
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About This Quiz

This USMLE microbiology quiz assesses knowledge on identifying pathogens, understanding their attributes, and diagnosing infections. It covers various bacteria and their characteristics relevant to medical scenarios, testing critical clinical and microbiological skills.

USMLE Microbiology Part 1 - Quiz

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  • 2. 

    A 15-year-old girl in a rural community has swollen, painful lymph nodes in her right axilla. Physical examination reveals multiple scratches on her right arm with a papule associated with one of the scratch marks. She states that the scratches occurred about 5 days ago. What type of animal is the most likely source of the infection?

    • Cat or kitten

    • Chicken

    • Dog or puppy

    • Horse

    • Parrot

    Correct Answer
    A. Cat or kitten
    Explanation
    The correct answer is A. This patient has the classic symptoms of cat scratch disease caused by the bacillus
    Bartonella henselae. The disease is self-limited with the onset of symptoms occurring 3-10 days following an
    inoculating scratch. The organism can be isolated from kittens, typically less than 1 year of age, or from fleas. A
    history of a new kitten in the house and the papule at the site of a scratch with regional painful adenopathy
    defines the classic scenario.
    Chickens (choice B) can harbor Salmonella spp. producing a gastroenteritis or enterocolitis. Chicken guano is
    also a favorable environment for the fungus Histoplasma capsulatum. The mycelial phase thrives in the rich soil.
    The human disease is a granulomatous infection involving the lungs and mimicking tuberculosis.
    Dogs or puppies (choice C) carry Capnocytophaga canimorsus as part of the normal flora of the oral cavity.Infections from licking or biting range from a self- limited cellulitis to fatal septicemia. Patients at risk for more
    severe infections are those with asplenia, alcoholism, or hematologic malignancies. This organism is also
    associated with cat bites, but the patient develops cellulitis and fulminant septicemia, especially in asplenic
    patients. Pasteurella multocida is another pathogen that colonizes the nasopharynx and gastrointestinal tract of
    cats and dogs. Cats have the highest rate of colonization (50-90%), followed by dogs (50%), swine (50%), and
    rats (14%). P. multocida most commonly causes a localized soft tissue infection or cellulitis after an animal bite,
    but systemic symptoms may be present in about 40% of the cases. These symptoms include osteomyelitis,
    septic arthritis, or tenosynovitis.
    Horses (choice D) and horse manure have been associated with a pulmonary opportunistic infection with
    cavitation caused by Rhodococcus equi that resembles tuberculosis in immunocompromised patients.
    Burkholderia mallei (the cause of glanders) is characterized by non-caseating granulomatous abscesses of
    skin, lymphadenopathy, and pronounced involvement of the lungs.
    Parrots (choice E) are associated with psittacosis caused by Chlamydia psittaci. Psittacosis is associated with a
    dry, hacking cough productive of scant sputum, an interstitial infiltrate in the lungs, severe headache, and
    myalgias. A pale macular rash is also seen.

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  • 3. 

    An African child develops massive unilateral enlargement of his lower face in the vicinity of the mandible. Biopsy demonstrates sheets of medium-sized blast cells with admixed larger macrophages. This type of tumor has been associated with which of the following?

    • Epstein-Barr virus and t(8;14)

    • Hepatitis B and t(9;22)

    • Herpesvirus and CD5

    • Human immunodeficiency virus and CD4

    • Human papillomavirus and t(2;5)

    Correct Answer
    A. Epstein-Barr virus and t(8;14)
    Explanation
    The correct answer is A. The patient has Burkitt's lymphoma. This type of lymphoma is a high-grade B-cell
    lymphoma that occurs in endemic form in Africa (it is the most common neoplasm in children in an equatorial
    belt that includes Africa and New Guinea) and sporadically in the United States and Europe. The sporadic form
    is often in an abdominal site and occurs in young adults. The African form of Burkitt's lymphoma has been
    strongly associated with antibodies directed against Epstein-Barr virus; the association is weaker in sporadic
    cases. A characteristic translocation, t(8;14) (q24.l3;q32.33) has been described.
    Hepatitis B (choice B) is associated with hepatocellular carcinoma. t(9;22) is the Philadelphia chromosome,
    which is seen in some cases of CML and AML.
    Herpesvirus (choice C) does not have a strong tumor association, although a link to cervical cancer has
    intermittently been proposed. CD5 is a marker seen in small lymphocytic and mantle cell lymphomas.
    HIV (choice D) is linked to Kaposi's sarcoma (and AIDS). Some patients also develop primary lymphomas (not
    usually Burkitt's). CD4 is a marker for helper T cells and some T cell lymphomas.
    Human papillomavirus (choice E) is linked with common warts, genital condylomata, and genital cancers. t(2;5)
    is linked to anaplastic large cell lymphoma.

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  • 4. 

    A 4-year-old boy is brought to the emergency room in extreme respiratory distress, with a temperature of 103.8 degrees Fahrenheit. He is drooling and has difficulty swallowing, and on physical examination, inspiratory stridor is noted. A lateral x-ray shows swelling of the epiglottis. He has had no previous vaccinations. Which of the following agents is the most likely cause of these symptoms?

    • Haemophilus influenzae

    • Klebsiella pneumoniae

    • Legionella pneumophila

    • Mycoplasma pneumoniae

    • Streptococcus pyogenes

    Correct Answer
    A. Haemophilus influenzae
    Explanation
    The correct answer is A. Epiglottitis is the most common disease of the upper respiratory tract produced by
    Haemophilus influenzae type b, a gram-negative encapsulated rod. It is also a common cause of otitis media in
    children and may cause bronchitis, bronchiolitis, and pneumonia in adults. The incidence of serious disease
    caused by Haemophilus influenzae type b has decreased greatly with the introduction of an effective vaccine.
    The vaccine is composed of the H. influenzae type b capsular polysaccharides coupled to a carrier molecule,
    given to children between 2 and 15 months of age. The patient had not received the Hib conjugate vaccine and
    therefore was susceptible to this organism.
    Klebsiella pneumoniae(choice B) causes pneumonia and pulmonary abscesses, but is not considered to be a
    pathogen in the upper respiratory tract.
    Legionella pneumophila(choice C) causes pneumonia in man. The disease may be mild (an atypical
    pneumonia) or a fulminating disease with a high mortality (30%).
    Mycoplasma pneumoniae(choice D) causes community-acquired atypical pneumonia. It is the most common
    cause of pneumonia in young adults.
    Streptococcus pyogenes(choice E) is the most common cause of pharyngitis; however, this patient's
    presentation strongly suggests epiglottitis.

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  • 5. 

    A 60-year-old alcoholic smoker abruptly develops high fever, shakes, a severe headache, and muscle pain. He initially has a dry, insignificant cough, but over the next few days he develops marked shortness of breath requiring assisted ventilation. Chest x-ray demonstrates homogeneous radiographic shadowing that initially involves the left lower lobe but continues to spread until both lungs are extensively involved. Culture of bronchoalveolar lavage fluid on buffered charcoal yeast extract (BCYE) demonstrates a coccobacillary pathogen. Which of the following is the most likely causative organism?

    • Legionella pneumophila

    • Listeria monocytogenes

    • Spirillium minus

    • Staphylococcus aureus

    • Streptococcus pneumoniae

    Correct Answer
    A. Legionella pneumophila
    Explanation
    The correct answer is A. The patient has a severe, potentially fatal, pneumonia with prominent systemic
    symptoms. Culture on BCYE is the specific clue that the organism is Legionella pneumophila. The disease is
    respiratory Legionellosis, also known as Legionnaire's disease, because the disease was first described when it
    occurred in epidemic form following an American Legion convention at a Philadelphia hotel. Patients tend to be
    older (40-70 years old) and may have risk factors including cigarette use, alcoholism, diabetes, chronic illness,
    or immunosuppressive therapy.
    Listeria monocytogenes(choice B) causes listeriosis and is not a notable cause of pneumonia.
    Spirillium minus (choice C) is a cause of rat-bite fever and is not a notable cause of pneumonia.
    Staphylococcus aureus(choice D) can cause pneumonia, but is easily cultured on routine media.
    Streptococcus pneumoniae(choice E) can cause pneumonia, but is easily cultured on routine media.

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  • 6. 

    A Pap smear from a woman with chronic cervicitis shows cytoplasmic inclusions within epithelial cells. Fluorescent antibodies identify both these inclusions and "elementary bodies." The intracellular organisms causing the infection are unusual because they cannot synthesize which of the following?

    • ATP

    • Cholesterol

    • DNA

    • Proteins

    • RNA

    Correct Answer
    A. ATP
    Explanation
    The correct answer is A. The disease is chlamydial cervicitis. This venereally transmitted infection is usually
    suspected after treatment for gonorrhea fails to relieve symptoms. However, a few cases are picked up when
    cytoplasmic inclusions composed of aggregates of chlamydia are identified on Pap smears. Confirmation can
    be made with fluorescent antibodies that pick up both the aggregates and individual bacteria known as
    "elementary bodies." The organisms are obligate intracellular parasites because they have lost the ability to
    synthesize ATP. Although some authors consider them to be "bacteria," others do not, since this is such a
    fundamental difference between the chlamydia and free living bacteria.
    Failure to synthesize cholesterol (choice B) is not usually cited as a problem specific to any type of organism,
    although viruses are not able to do so without using host machinery.
    Some viruses cannot synthesize DNA (choice C), but the disease in question is not caused by a virus.
    Protein synthesis (choice D) by viruses requires host ribosomes.
    Some viruses cannot synthesize RNA (choice E), but the disease in question is not caused by a virus.

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  • 7. 

    One of three blood culture bottles drawn from a patient with unexplained fevers reveals gram-positive cocci growing in clusters. Which of the following tests would be most useful in determining whether this organism is a part of the normal skin flora?

    • Bacitracin resistance

    • Catalase

    • Coagulase

    • Novobiocin resistance

    • Optochin resistance

    Correct Answer
    A. Coagulase
    Explanation
    The correct answer is C. Gram-positive cocci in clusters are staphylococci. Staphylococcus aureus is a common
    pathogen that should not be considered normal skin flora. Other staphylococci, especially Staphylococcus
    epidermidis, may contaminate blood cultures and can be differentiated from Staphylococcus aureus by the
    coagulase test. S. aureus is the only coagulase-positive staphylococcus.
    Bacitracin sensitivity differentiates Streptococcus pyogenes from the other beta-hemolytic streptococci, which
    are bacitracin resistant (choice A).
    The catalase test (choice B) is used to differentiate streptococci from staphylococci. Staphylococci produce
    catalase and can generate oxygen bubbles in hydrogen peroxide, whereas streptococci cannot.
    Novobiocin resistance (choice D) differentiates the coagulase-negative staphylococci into S. epidermidis
    (novobiocin sensitive) and S. saprophyticus (novobiocin resistant).
    Optochin resistance (choice E) differentiates the major pathogenic alpha-hemolytic streptococci. S. pneumoniae
    is optochin- and bile-sensitive whereas S. viridans is resistant to both optochin and bile.

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  • 8. 

    A 49-year-old Vietnamese man is diagnosed with tuberculosis. On physical examination, large flocculent masses are noted over the lateral lumbar back, and a similar mass is located in the ipsilateral groin. This pattern of involvement strongly suggests an abscess tracking along the

    • Adductor longus

    • Gluteus maximus

    • Gluteus minimus

    • Piriformis

    • Psoas major

    Correct Answer
    A. Psoas major
    Explanation
    The correct answer is E. This is the classic presentation of a psoas abscess. This clinical entity was formerly a
    fairly common complication of vertebral tuberculosis, but is now rare in clinical practice in this country. The
    psoas muscle is covered by a fibrous sheath known as the psoas fascia. This sheath is open superiorly,
    permitting an infection involving the soft tissues around the spine to enter the sheath, then track down to the
    groin.
    The adductor longus (choice A) is a muscle of the anterior thigh, and is not related to the lumbar portion of the back.
    The gluteus maximus (choice B) gluteus minimus (choice C) and piriformis (choice D) are muscles of the
    buttock with no relationship to the groin.

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  • 9. 

    A newborn is infected in utero with an enveloped virus containing double-stranded DNA. The child develops petechiae, hepatosplenomegaly, and jaundice. Brain calcifications are detected on computed tomography (CT). With which of the following viruses is the newborn most likely infected?

    • Cytomegalovirus

    • Hepatitis B

    • Hepatitis C

    • Hepatitis D

    • Herpes simplex

    Correct Answer
    A. Cytomegalovirus
    Explanation
    The correct answer is A. The most common agents causing congenital infection in the United States are
    cytomegalovirus, herpes simplex virus, and Treponema pallidum. Congenital cytomegalovirus infection, which
    occurs in 1 to 2% of all live births in the U.S., results from transplacental acquisition of a primary or recurrent
    maternal infection. Most infections are asymptomatic, but about 5% will present with hepatosplenomegaly,
    periventricular brain calcification, petechial hemorrhages, and hydrops. Sensorineural deafness is a common
    manifestation in affected infants. Primary infection of the mother is strongly associated with more severe
    symptoms in affected infants. The mortality rate is 30% and most of the survivors will have neurologic
    impairment. CMV infections in children and adults are often asymptomatic; however, the virus is responsible for 10% of infectious mononucleosis cases (characterized by an absence of sheep RBC agglutinins). CMV also
    causes serious disease in immunosuppressed patients, including bone marrow recipients and AIDS patients.
    Ganciclovir is the drug of choice, although foscarnet has also been used with some success.
    Infections with hepatitis viruses (choices B, C, and D), if symptomatic, are characterized by jaundice, lethargy,
    failure to thrive, abdominal distention, clay- colored stools, and elevated transaminase levels. Hepatitis B(choice
    B) is the only viral hepatitis agent that is recognized as an important cause of neonatal disease. The infection is
    usually acquired during the birth process; therapy includes hyperimmune human anti-hepatitis B serum followed
    a few weeks later with active immunization with rHBsAg injections.
    Herpes simplex (choice E) can also infect the neonate. The hallmark of infection is painful skin vesicles (present
    in 50% of affected infants); other manifestations include encephalitis, pneumonia, hepatitis, and disseminated
    intravascular coagulopathy. Massive hepatic and adrenal necrosis is common. Therapy is with acyclovir.

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  • 10. 

    A patient with colorectal cancer develops septicemia complicated by endocarditis. You would expect the blood cultures to grow

    • Streptococcus agalactiae

    • Streptococcus bovis

    • Streptococcus pneumoniae

    • Streptococcus pyogenes

    • Streptococcus viridans

    Correct Answer
    A. Streptococcus bovis
    Explanation
    The correct answer is B.Streptococcus bovis is a Group D streptococcus. There is a significant association
    between S. bovis bacteremia and endocarditis with carcinoma of the colon and other colonic diseases. Every
    patient with S. bovis bacteremia should undergo gastrointestinal and cardiac evaluation. Up to 50% of patients
    with S. bovis bacteremia are reported to have underlying colonic malignancies. In another study, 25-50% of
    cases of S. bovis bacteremia were associated with endocarditis, especially in patients with preexisting valvular
    lesions.
    Streptococcus agalactiae(choice A) is an important cause of maternal and neonatal bacteremia and neonatal
    meningitis. It is part of the normal flora of the gastrointestinal tract and the female genital tract.
    Streptococcus pneumoniae(choice C) is a leading cause of community-acquired pneumonia, meningitis in
    adults, otitis media (especially in children), and sinusitis. Spontaneous peritonitis due to S. pneumoniae is
    reported in children with ascites from nephrotic syndrome. Asplenia predisposes patients to severe infections
    with S. pneumoniae and other encapsulated organisms. S. pneumoniae infections are also more frequent and
    unusually severe in patients with sickle cell anemia, multiple myeloma, alcoholism, and hypogammaglobulinemia.
    S. pneumoniae is now the leading cause of invasive bacterial respiratory disease in patients with AIDS.
    Streptococcus pyogenes(choice D) is the most common cause of bacterial pharyngitis. Complications include
    paratonsillar abscesses, otitis media, and sinusitis. Long-term sequelae include rheumatic fever and
    poststreptococcal glomerulonephritis. S. pyogenes is also responsible for many skin and soft tissue infections.
    The organism also produces many toxins that produce a variety of diseases.
    Streptococcus viridans(choice E), or the viridans Streptococci, are the most common cause of subacute
    bacterial endocarditis, which should be suspected in cases of viridans streptococcal bacteremia. One species of
    viridans Streptococci, Streptococcus milleri, is frequently associated with pyogenic abscesses, especially of the
    liver.

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  • 11. 

    A 37-year-old female presents to the emergency room with a fever. Chest x-ray shows multiple patchy infiltrates in both lungs. Echocardiography and blood cultures suggest a diagnosis of acute bacterial endocarditis limited to the tricuspid valve. Which of the following is the most probable etiology?

    • Congenital heart disease

    • Illicit drug use

    • Rheumatic fever

    • Rheumatoid arthritis

    • Systemic lupus erythematosus

    Correct Answer
    A. Illicit drug use
    Explanation
    The correct answer is B. The most probable etiology of bacterial endocarditis involving the tricuspid valve is
    illicit intravenous drug use, which can introduce skin organisms into the venous system that then attack the
    tricuspid valve. Staphylococcus aureus accounts for between 60% and 90% of cases of endocarditis in
    intravenous drug users.
    The endocarditis associated with congenital heart disease (choice A) typically involves either damaged valves
    or atrial or ventricular septal defects. The tricuspid valve is not particularly vulnerable.
    Rheumatic fever (choice C) most commonly damages the mitral and aortic valves, and tricuspid damage is
    usually less severe and seen only when the mitral and aortic valves are heavily involved. Consequently,
    secondary bacterial endocarditis involving only the tricuspid valve in a patient with a history of rheumatic fever
    would be unusual.
    Rheumatoid arthritis (choice D) is not associated with bacterial endocarditis.
    Systemic lupus erythematosus (choice E) can produce small, aseptic vegetations on valves, but is not
    associated with bacterial endocarditis.

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  • 12. 

    A 16-year-old girl presents with a painlessly enlarged lymph node in her right axilla. Peripheral blood counts are within normal limits. The lymph node is biopsied, and numerous granulomas filled with neutrophils and necrotic debris are observed. Which of the following organisms could produce this disease?

    • Bartonella henselae

    • Borrelia burgdorferi

    • Chlamydia psittaci

    • Coxiella burnetii

    • Rickettsia prowazekii

    Correct Answer
    A. Bartonella henselae
    Explanation
    The correct answer is A.Bartonella henselae is the infective agent of cat scratch disease, which generally
    presents as regional lymphadenopathy with or without low fevers and headaches. Bartonella is a gram-variable
    pleomorphic rickettsial organism that is introduced to the skin in a cat bite or scratch. It produces a self-limited
    granulomatous response in the draining lymph nodes.
    Borrelia burgdorferi (choice B) is a spirochetal organism that is transmitted by a tick bite (Ixodes spp.),
    producing Lyme disease. Lyme disease progresses from a skin rash to fevers, headache and pain over about
    one month. It may produce lymphadenopathy, but is not associated with granuloma formation.
    Chlamydia psittaci (choice C) infection occurs after contact with infected bird droppings and produces an
    atypical pneumonia. The central nervous system may also be involved, but lymph nodes are spared. Chlamydia
    trachomatis is the chlamydial species that typically produces suppurative nodal granulomas (lymphogranuloma
    venereum).
    Coxiella burnetii(choice D) infection is transmitted by inhaling dusts or drinking milk from infected mammals,
    especially sheep and cows. The disease in humans, Q fever, is marked by mild nonspecific symptoms or
    pneumonia, and may progress to myocarditis or hepatitis.
    Rickettsia prowazekii(choice E) produces epidemic (louse-borne) typhus, which is transmitted by body lice and
    produces a rash akin to Rocky Mountain spotted fever. Although the organism may reside in the lymph nodes in
    dormancy, it does not elicit granuloma formation.

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  • 13. 

    A very ill neonate has widespread granulomas. In utero infection with which of the following organisms is suggested by this finding?

    • Clostridium botulinum

    • Escherichia coli

    • Haemophilus influenzae

    • Listeria monocytogenes

    • Neisseria gonorrhoeae

    Correct Answer
    A. Listeria monocytogenes
    Explanation
    The correct answer is D. All of the organisms listed can cause infection in infants, but Listeria monocytogenes is
    the one to cause potentially fatal granulomas (granulomatis infantiseptica) following in utero infection.
    Clostridium botulinum(choice A) can cause infant botulism, characterized by failure to thrive, and progressive
    muscular weakness.
    Escherichia coli(choice B) can cause diarrhea, pneumonia, and meningitis in infants.
    Haemophilus influenzae(choice C) can cause acute epiglottitis, meningitis, pneumonia, and otitis media in
    young children.
    Neisseria gonorrhoeae(choice E) can be transmitted via the birth canal, and causes ophthalmia neonatorum.

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  • 14. 

    A 54-year-old woman suffering from influenza deteriorates and develops shaking chills and a high fever. Physical examination is remarkable for dullness to percussion at the left base and decreased breath sounds on the left. Chest x-ray confirms the diagnosis of lobar pneumonia, presumed to be caused by Streptococcus pneumoniae. The patient has no known drug allergies. Which of the following antibiotics would be most appropriate to treat the patient's condition?

    • Cefotaxime

    • Chloramphenicol

    • Erythromycin

    • Penicillin

    • Vancomycin

    Correct Answer
    A. Penicillin
    Explanation
    The correct answer is D. Penicillin remains the first-line drug of choice for pneumococcal pneumonia, except in
    patients with penicillin allergy and in the relatively few areas in which pneumococcal strains with high-level
    penicillin resistance exist. Alternative therapies include erythromycin and vancomycin.
    The third-generation cephalosporin cefotaxime (choice A) is not usually used for pneumococcal pneumonia.
    Chloramphenicol (choice B) is not usually used for pneumococcal pneumonia.
    Erythromycin (choice C) is a good alternative therapy for pneumococcal pneumonia, but is usually used only
    when a penicillin allergy is present.
    Vancomycin (choice E) is not the first-line therapy, but it is a good alternative in patients allergic to penicillin or
    when high-level penicillin resistance (relatively uncommon) is present.

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  • 15. 

    An otherwise healthy 3-year-old child is brought to the pediatrician with umbilicated, flesh-colored papules on his trunk. This condition is related to infection with which of the following viruses?

    • Cytomegalovirus

    • Herpesvirus 6

    • Parvovirus

    • Poxvirus

    • Variola

    Correct Answer
    A. Poxvirus
    Explanation
    The correct answer is D. The lesions are characteristic of molluscum contagiosum, which is a typically benign
    and self-limited condition caused by a poxvirus. The disease can be transmitted either venereally or through
    non-venereal contact. The other viruses listed do not cause similar skin lesions. Patients with advanced HIV
    infection may develop a severe, generalized, and persistent eruption, often involving the face and upper body.
    Cytomegalovirus (choice A) causes congenital infections and disseminated infections in immunosuppressed
    patients.
    Herpesvirus 6 (choice B)causes roseola (exanthem subitum).
    Parvovirus (choice C) causes aplastic crises in patients with hemolytic anemia.
    Variola (choice E) is the smallpox virus.

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  • 16. 

    A 37-year-old woman presents with 3 days of progressive joint pain in her ankles, knees, and wrists. She recalls three similar episodes over the past several years. On examination, she has a temperature of 38.7 C, her blood pressure is 110/70 mm Hg, and her heart rate is 90/min. She has a diffuse petechial rash over her trunk and extensor surfaces. Her ankles and knees are swollen, red, and tender with decreased range of motion, and there is tenderness over the tendon sheaths of her hands and forearms. Blood cultures are negative. Aspiration of joint fluid reveals a white cell count of 22,000/mm3 with no visible organisms, but culture on chocolate agar is positive. Which of the following is an attribute of the causative organism that allows it to produce recurrent infections?

    • It is an intracellular pathogen

    • It is resistant to ceftriaxone

    • It is resistant to complement-mediated lysis

    • Its capsule is not immunogenic

    • Its pili undergo antigenic and phase variation

    Correct Answer
    A. Its pili undergo antigenic and phase variation
    Explanation
    The correct answer is E. This is a case of Neisseria gonorrhoeae arthritis. Patients are continuously susceptible
    to reinfection because of antigenic variation and phase variation of the pili.
    N. gonorrhoeae is not an intracellular pathogen (choice A), although it may be found intracellularly in
    neutrophils after it has been phagocytized.
    Ceftriaxone (choice B) is the drug of choice for N. gonorrhoeae.
    Gonococci are especially susceptible to complement-mediated lysis, not resistant to it (choice C).
    N. gonorrhoeae has an insignificant capsule that does not play a major role in the pathogenesis, but its capsule
    is immunogenic (compare with choice D). Streptococcus pyogenes is the best known example of a
    nonimmunogenic capsule, made of hyaluronic acid, but virtually all other capsules are immunogenic

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  • 17. 

    A 73-year-old woman with a history of diabetes presents with left ear pain and drainage of pus from the ear canal. She has swelling and tenderness over the left mastoid bone. Which of the following microorganisms is the most likely causative agent?

    • Hemophilus influenzae

    • Klebsiella pneumoniae

    • Mucor sp

    • Pseudomonas aeruginosa

    • Streptococcus pyogenes

    Correct Answer
    A. Pseudomonas aeruginosa
    Explanation
    The correct answer is D.Pseudomonas aeruginosa causes malignant otitis externa, which is a severe
    necrotizing infection of the external ear canal. Infection tends to spread to the mastoid bone, temporal bone,
    sigmoid sinus, base of the skull, meninges, and brain. Patients at increased risk include the elderly, those with
    diabetes, and the immunocompromised. Pseudomonas is also associated with many other clinical syndromes,
    including infection following traumatic wounds to the feet in persons wearing sneakers or rubber-soled shoes,
    since the organism can be cultured from the shoes.
    Hemophilus influenzae(choice A) produces a variety of clinical syndromes. H.Influenzae is the third-most
    common cause of meningitis in children aged 1 month to 18 years. It is the most common cause of acute
    epiglottitis, the most common cause of purulent bacterial conjunctivitis, and the second-most common cause of
    otitis media. Patients at risk include those with COPD and cystic fibrosis, alcoholics, splenectomized patients,
    and young patients.
    Klebsiella pneumoniae(choice B) is a gram-negative organism that produces a necrotizing pneumonia in
    diabetics and alcoholics. Patients typically present with an abrupt onset of fever, shaking chills, and purulent,
    foul-smelling sputum.
    Mucor (choice C) is a fungal infection that is particularly severe in the diabetic or the immunocompromised
    patient. In the acidotic diabetic, the fungus produces a life-threatening, invasive rhinocerebral infection. The
    infection begins in the nasal passages, extends into the paranasal sinuses, and spreads through the cribiform
    plate to the frontal lobes of the brain. Patients typically complain of headache, facial pain, and orbital swelling.
    Streptococcus pyogenes(choice E) causes bacterial pharyngitis, otitis media, and sinusitis. It is also associated with toxin-related diseases and skin infections.

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  • 18. 

    A 29-year-old female diagnosed with AIDS has been suffering from a progressive blurring of vision in her right eye. On funduscopic examination, a small white opaque lesion is noted on the retina of her right eye. Which of the following is the most appropriate therapy for this patient?

    • Acyclovir

    • Amantadine

    • Flucytosine

    • Ganciclovir

    • Zidovudine

    Correct Answer
    A. Ganciclovir
    Explanation
    The correct choice is D. This patient is most likely suffering from cytomegalovirus (CMV) retinitis. The best drug
    treatment for this infection is ganciclovir.
    Acyclovir (choice A) is not effective in CMV infections. It is used more for HSV type 1 and 2 infections.
    Amantadine (choice B) is used either therapeutically or prophylactically for the influenza A virus.
    Flucytosine (choice C) is an antifungal agent.
    Zidovudine (choice E) is a first-line drug for the treatment of AIDS. The drug by itself is ineffective against CMV retinitis.

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  • 19. 

    A 36-year-old man with AIDS develops right-sided weakness involving the lower, but not the upper, limb. MRI scans reveal a ring-enhancing lesion within the white matter of the left frontal lobe. A biopsy shows coagulative necrosis of brain parenchyma with macrophage-rich chronic inflammatory infiltration admixed with microscopic cysts that contain characteristic bradyzoites. Which of the following is the most common source of this type of infection?

    • Anopheles mosquitoes

    • Bird droppings

    • Cats

    • Cooling systems

    • Washbasins

    Correct Answer
    A. Cats
    Explanation
    The correct answer is C. Clues to the correct answer are the underlying disease (AIDS), which predisposes
    to opportunistic infections, the typical MRI appearance of the lesion (ring-enhancing mass), and the
    histopathologic features (presence of encysted bradyzoites). In short, this patient has cerebral toxoplasmosis,
    which represents one of the most common opportunistic infections in AIDS. Toxoplasma gondii is a protozoon
    that infects humans who ingest the oocysts from cat feces or incompletely cooked lamb or pork. Only
    immunodepressed patients and fetuses are vulnerable to this infection. In the fetus, toxoplasmosis causes
    extensive damage to brain parenchyma and retina. Toxoplasmosis associated with AIDS manifests with
    necrotizing lesions surrounded by chronic inflammation. A ring-enhancing lesion is a mass that contains a rim of
    contrast enhancement (bright signal on MRI) surrounding a dark core corresponding to central necrosis. In
    AIDS, the most frequent causes of a ring-enhancing lesion are primary brain lymphoma and toxoplasmosis.
    Anopheles mosquitoes (choice A) transmit malaria parasites. Cerebral malaria is caused by Plasmodium
    falciparum, which is able to adhere to endothelial cells of small cerebral vessels. Vascular occlusion ensues,
    resulting in numerous small infarcts.
    Bird droppings (choice B) represent the vehicle of infection for two of the most common opportunistic fungal
    infections affecting immunocompromised patients: cryptococcosis and histoplasmosis. Cryptococcus
    neoformans causes meningoencephalitis, not intracerebral necrotic lesions. Histoplasma capsulatum rarely
    affects the brain. These fungi can be visualized in tissue sections by silver stains.
    Cooling systems (choice D) may harbor Legionella pneumophila, spreading the bacteria in aerosolized form. L. pneumoniae is a gram-negative bacterium that causes Legionnaire's disease, a fatal form of pneumonia that
    first struck participants at a meeting of the American Legion. It has been reported in immunocompromised
    patients as well.
    Washbasins (choice E) frequently contain Pseudomonas aeruginosa, which has also been isolated from
    respirator devices, cribs, and antiseptic-containing bottles. P. aeruginosa tends to affect patients with cystic
    fibrosis, severe burns, or immune impairment. It may cause bronchopneumonia, osteomyelitis, endocarditis,
    external otitis, and keratitis, but not cerebral infection.

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  • 20. 

    A newborn infant has multiple, hemorrhagic, cutaneous lesions and does not respond to sound. Head CT scan shows periventricular calcifications. Which of the following infectious agents is the most likely cause of this child's presentation?

    • Cytomegalovirus

    • Herpes simplex

    • Rubella

    • Syphilis

    • Toxoplasmosis

    Correct Answer
    A. Cytomegalovirus
    Explanation
    The correct answer is A. The infectious agents listed are all important causes of congenital disease. The triad of
    cutaneous hemorrhages ("blueberry muffin baby"), deafness, and periventricular CNS calcifications suggests
    congenital CMV infection, the most common cause of intrauterine fetal viral infection. Other manifestations
    include microcephaly and hepatosplenomegaly.
    Neonatal herpes (choice B) may be congenital, but more commonly is acquired during vaginal delivery. The
    infection is characterized by vesicles on the skin and mucous membranes, encephalitis, or disseminated
    disease.
    Congenital rubella (choice C) can cause mental retardation, heart abnormalities, blindness, encephalitis, and
    motor abnormalities.
    Congenital syphilis (choice D) can cause death in utero, or a variety of problems including abnormal teeth,
    bones, and central nervous system.
    Toxoplasmosis (choice E) can be either acquired during delivery (mild) or congenital (severe). Severe infections
    can cause stillbirth, chorioretinitis, intracerebral calcifications, and hydro- or microcephaly.

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  • 21. 

    A 33-year-old woman presents with fever, vomiting, severe irritative voiding symptoms, and pronounced costovertebral angle tenderness. Laboratory evaluation reveals leukocytosis with a left shift; blood cultures indicate bacteremia. Urinalysis shows pyuria, mild hematuria, and gram-negative bacteria. Which of the following drugs would best treat this patient's infection?

    • Ampicillin and gentamicin

    • Erythromycin

    • Gentamicin and vancomycin

    • Phenazopyridine and nitrofurantoin

    • Tetracycline

    Correct Answer
    A. Ampicillin and gentamicin
    Explanation
    The correct answer is A. Acute pyelonephritis is an infectious disease involving the kidney parenchyma and the
    renal pelvis. Gram-negative bacteria, such as Escherichia coli, Proteus, Klebsiella, and Enterobacter, are the
    most common causative organisms in acute pyelonephritis. Laboratory evaluation will often reveal leukocytosis
    with a left shift, and urinalysis typically shows pyuria, varying degrees of hematuria, and white cell casts. Since
    bacteremia is present, the patient should be hospitalized and empirically started on IV ampicillin and gentamicin.
    This regimen may be need to be changed, however, once the sensitivity results are available.
    Erythromycin (choice B) and tetracycline (choice E) are both bacteriostatic antibiotics and would not be
    recommended in a patient with a severe infection, such as acute pyelonephritis with bacteremia.
    Vancomycin (choice C) is primarily used in the treatment of severe gram-positive infections.
    Phenazopyridine (choice D) is a urinary analgesic, and nitrofurantoin (choice D) is a urinary tract anti-infective.
    Although nitrofurantoin is indicated for the treatment of "mild" cases of pyelonephritis, as well as cystitis, this
    patient's condition is severe and should be treated with appropriate antibiotics.

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  • 22. 

    A 15-year-old boy presents to his physician with several weeks of slowly worsening pruritus of both of his feet. He is otherwise well and taking no medications. On examination, he has bilateral, erythematous, dry scaling lesions that are most obvious in the interdigital web spaces and on the soles. There is no bleeding or exudate. What would most likely be found in a potassium hydroxide (KOH) mount of a scraping of the affected skin?

    • Branching hyphae with rosettes of conidia

    • Branching, septate hyphae

    • Budding yeasts

    • Hyphae, arthroconidia, and blastoconidia

    • Pigmented, septate hyphal fragments

    • Short, curved hyphae and round yeasts

    Correct Answer
    A. Branching, septate hyphae
    Explanation
    The correct answer is B. This is a case of tinea pedis, or athlete's foot, caused by a variety of dermatophytic
    fungi, which are easily detected on alkali mounts of scraped skin as colorless, branching hyphae with
    cross-walls.
    Hyphae with rosettes of conidia (choice A) describes the environmental/transmission form for Sporothrix
    schenckii, the agent of rose gardener's disease, which is a subcutaneous mycosis.
    Budding yeasts (choice C) describes the form found in clinical specimens from patients with sporotrichosis.
    Hyphae, arthroconidia, and blastoconidia (choice D) would be found in clinical specimens from patients infected
    with Trichosporon beigelii (white piedra), which is a superficial mycosis of the hair of the head.
    Pigmented, septate hyphal fragments (choice E) would be found in cases of phaeohyphomycosis, a diverse
    group of cyst-forming subcutaneous, pigmented (dematiaceous) fungi, rare in the U.S.
    Short, curved hyphae and round yeasts (choice F), or the "spaghetti and meatball" presentation in clinical
    specimens, are characteristic of Malassezia furfur, the agent of pityriasis versicolor.

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  • 23. 

    A 35-year-old woman presents to her gynecologist with complaints of burning on urination for the past 2 days. Dipstick test of her urine demonstrates marked positivity for leukocyte esterase, but no reactivity for nitrite. Urine culture later grows out large numbers of organisms. Which of the following bacteria are most likely to be responsible for this patient's infection?

    • Enterobacter sp.

    • Enterococcus faecalis

    • Escherichia coli

    • Klebsiella pneumoniae

    • Pseudomonas aeruginosa

    Correct Answer
    A. Enterococcus faecalis
    Explanation
    The correct answer is B. The positive leukocyte esterase test indicates the presence of neutrophils in the urine,
    suggesting a bacterial infection. The nitrite test exploits the fact that most Enterobacteria (gram-negative enteric
    rods) are able to form nitrite from nitrate; thus, the nitrite test is used to diagnose urinary tract infections. One
    limitation of this method is the fact that enterococci (gut streptococci) do not produce nitrite from nitrate, but can
    nonetheless cause urinary tract infections. Enterococcal urinary tract infections are often nosocomial and
    classically acquired in the intensive care unit, although they can occur in other settings.
    Enterobacter sp. (choice A), Escherichia coli(choice C), Klebsiella pneumoniae(choice D), and Pseudomonas
    aeruginosa(choice E) can cause urinary tract infections and would usually be picked up by the dipstick for
    nitrites. False-negative results might still be seen with these organisms if the infection was light, the bladder had
    been recently emptied prior to collection, and the urine was "new" and had not yet grown enough bacteria to
    produce a positive result.

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  • 24. 

    A 39-year-old black man presents with complaints of anorexia, malaise, fatigue, dark urine, and upper abdominal discomfort. He admits to homosexuality, but denies blood transfusions, alcohol intake, or intravenous drug abuse. On physical examination, the patient has a temperature of 100.2 degrees F, scleral icterus, and jaundice. His liver is palpable below the right costal margin, and there is moderate right upper quadrant tenderness. Liver function test results are as follows: total bilirubin 12.4%, SGOT 980 units, SGPT 1200 units. Serologic findings are as follows: anti-hepatitis A IgM negative, HBsAg positive, anti-HBc IgM positive, HBeAg positive, anti-HBsAb negative, hepatitis C negative. When this patient enters the window period, what would likely be the first change in his serologic findings?

    • He will become HBcAg positive

    • He will become HBc IgG positive

    • He will become HBeAg negative

    • He will become HBsAb positive

    • He will become HBsAg negative

    Correct Answer
    A. He will become HBsAg negative
    Explanation
    The correct answer is E. This patient has acute hepatitis B. The "window period" refers to that period in
    infection when neither hepatitis B surface antigen (HBsAg) nor its antibody (HBsAb) can be detected in the
    serum of the patient. It is an immunologically mediated phenomenon caused by the precipitation of
    antigen-antibody complexes in their zone of equivalent concentrations and, thereby, their removal from the
    circulation. Because of this, the first thing that will happen in the window period is that the serum will become
    negative for the surface antigen (HBsAg), as that antigen is precipitated out of the serum by developing levels
    of its specific antibody (HBsAb).
    HBcAg is not typically measured (choice A) in the serum.
    Levels of the c-core antibody HBc IgG (choice B) and HBeAg (choice C) do not have a relationship to the
    window period.
    Levels of HBsAb (choice D) will not be detectable until there is antibody excess, and the patient is leaving the
    window period.

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  • 25. 

    A 24-year-old AIDS patient develops chronic abdominal pain, low-grade fever, diarrhea, and malabsorption. Oocysts are demonstrated in the stool. Which of the following organisms is most likely to be the cause of the patient's diarrhea?

    • Diphyllobothrium latum

    • Entamoeba histolytica

    • Giardia lamblia

    • Isospora belli

    • Microsporidia

    Correct Answer
    A. Isospora belli
    Explanation
    The correct answer is D. All of the organisms listed are protozoa. There are two intestinal protozoa specifically
    associated with AIDS that can cause transient diarrhea in immunocompetent individuals but can cause
    debilitating, and potentially life-threatening chronic diarrhea in AIDS patients. These organisms are Isospora
    belli, treated with trimethoprim-sulfamethoxazole (or other folate antagonists) and Cryptosporidium parvum (no
    treatment presently available).
    Diphyllobothrium latum(choice A) is the fish tapeworm and occasionally causes diarrhea.
    Entamoeba histolytica(choice B) and Giardia lamblia(choice C) are both causes of diarrhea, but they are not
    specifically associated with AIDS.
    Microsporidia (choice E) are a protozoan cause of diarrhea, but produce spores rather than oocysts.

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Stephen Reinbold |PhD (Biological Sciences) |
Biology Instructor
Stephen Reinbold has a PhD in Biological Sciences and a strong passion for teaching. He taught various subjects including General Biology, Environmental Science, Zoology, Genetics, and Anatomy & Physiology at Metropolitan Community College in Kansas City, Missouri, for nearly thirty years. He focused on scientific methodology and student research projects. Now retired, he works part-time as an editor and engages in online activities.

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