USMLE Microbiology - Part 2
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Questions and Answers
- 1.
A 4-year-old boy presents to the pediatric emergency department with the classic meningitis triad of fever, headache, and nuchal rigidity. A lumbar puncture is performed and analysis of the fluid shows an increase in polymorphonuclear cells, an increased protein level, and a decreased glucose level. Which of the following is the most common cause of meningitis in a child of this age with this clinical picture?
- A.
Enteroviruses
- B.
Haemophilus infl uenzae type B
- C.
Herpes simplex virus
- D.
Listeria species
- E.
Streptococcus pneumoniae
Correct Answer
E. Streptococcus pneumoniaeExplanation
The correct answer is E. The results of the fluid analysis are consistent with a bacterial meningitis
(increased polymorphonuclear cells, high protein, low glucose). The most common cause of meningitis in children aged 6 months to 6 years is Streptococcus pneumoniae.
Answer A is incorrect. Results of the fluid analysis are not consistent with a viral meningitis. One would expect to see increased lymphocyte counts, normal protein levels, and normal sugar levels in this type of infection.
Answer B is incorrect. Haemophilus influenzae meningitis is a less common cause of meningitis in children of this age group. Since the introduction of the Haemophilus flu vaccine, the incidence of this cause of meningitis has greatly decreased.
C is incorrect. The clinical picture does not fit with a viral meningitis. In a viral meningitis, one would expect the fluid analysis to come back with increased lymphocytes, normal protein, and normal sugar.
Answer D is incorrect. Listeria species is not a common cause of meningitis in this age group. It is much more commonly seen in newborns age 0–6 months and in the elderly.Rate this question:
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- 2.
A 32-year-old man presents to his doctor with painful urination and a purulent urethral discharge. The image shows cells that have been cultured from this discharge. Which of the following is the treatment of choice for this infection?
- A.
Azithromycin
- B.
Ceftriaxone
- C.
Fluconazole
- D.
Penicillin
- E.
Vancomycin
Correct Answer
A. AzithromycinExplanation
The correct answer is A. These symptoms are typical of urethritis. The most common causes of urethritis in males are Chlamydia trachomatis and Neisseria gonorrhoeae. The image shows intracellular inclusions that indicate that this man is infected with C. trachomatis; in the image, EB indicates the elementary body particles within cell walls and RB indicates the reticular body. While they may be difficult to differentiate, C. trachomatis infection induces a predominantly immunologic reaction with only some polymorphonuclear leukocytes (PMNs), while N. gonorrhoeae induces predominantly nonimmunologic inflammation with a PMN-rich infi ltrate. The treatment of choice for Chlamydia urethritis is azithromycin (macrolide family antibiotic) or doxycycline (tetracycline family antibiotic).
Answer B is incorrect. Ceftriaxone is an effective treatment for gonorrhea, but the cephalosporin class of antibiotics is relatively ineffective against Chlamydia trachomatis.
Answer C is incorrect. Fluconazole inhibits fungal steroid synthesis. It is used in the treatment of
fungal infections, such as Candida albicans.
Answer D is incorrect. Penicillin has been shown to suppress chlamydial multiplication. However, it does not eradicate the organism and thus is not the best treatment for this type of infection. Penicillin is the treatment of choice for syphilis.
Answer E is incorrect. Vancomycin has not been shown to be effective in the treatment of chlamydial infection. It is used to treat drugresistant Staphylococcus aureus and Clostridiumdifficile.Rate this question:
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- 3.
A 43-year-old man presents with fl u-like symptoms, fevers, chills, and a productive cough. Physical examination is remarkable for pleuritic chest pain. On questioning, the patient says that he has just spent the last week on vacation in Central America. Cultures taken from the patient show a broad-based budding fungus. It is concluded that the man has systemic blastomycosis infection. Which of the following agents is the most appropriate treatment for this patient?
- A.
Amphotericin B
- B.
Fluconazole
- C.
Itraconazole or potassium iodide
- D.
Sodium stibogluconate only
- E.
Topical miconazole or selenium sulfide
Correct Answer
A. AmpHotericin BExplanation
The correct answer is A. Blastomycosis can present with fl u-like symptoms, fevers, chills, productive cough, myalgia, arthralgia, and pleuritic chest pain. Some patients will fail to recover from an acute infection and progress to develop chronic pulmonary infection or widespread disseminated infection. Fluconazole or ketoconazole is used for the treatment of local blastomycosis infections, and amphotericin B is used for the treatment of systemic infections.
Answer B is incorrect. Fluconazole or ketoconazole are effective treatments for local blastomycosis infections but are ineffective if the infection is systemic. Systemic infections require amphotericin B.
Answer C is incorrect. Itraconazole or potassium iodide is used for the treatment of Sporothrix
schenckii. S. schenckii is the cause of sporotrichosis. When S. schenckii is introduced into the skin, usually by a thorn prick, it causes a local pustule or ulcer with nodules along draining lymphatics (ascending lymphangitis). S. schenckii is a dimorphic fungus that has cigar-shaped budding yeast visible in pus.
Answer D is incorrect. Sodium stibogluconate is used to treat Leishmania donovani infection.
L. donovani presents with hepatomegaly and splenomegaly, malaise, anemia, and weight loss.
L. donovani is transmitted via the sandfly. Microscopically, macrophages containing amastigotes are observed.
Answer E is incorrect. Topical miconazole or selenium sulfide (Selsun) is used to treat Malassezia furfur. M. furfur is the cause of tinea versicolor. Symptoms of this infection include hypopigmented skin lesions that occur in hot and humid conditions.Rate this question:
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- 4.
73-year-old woman steps on a rusty nail while gardening in her back yard. A neighbor drives her to the emergency department, where her wound is cleaned and bandaged. A complete history and review of her medical chart indicates that her vaccinations are not up to date, and she requires one vaccination and a shot of the appropriate immunoglobulin as prophylaxis against infection. What is the mechanism of toxicity of the organism for which she was vaccinated?
- A.
ADP-ribosylation of a G protein, increasing chloride secretion
- B.
ADP-ribosylation of an elongation factor, causing disrupted protein synthesis
- C.
Binding to the MHC II receptor and T-lymphocyte receptor, causing cytokine synthesis
- D.
Blocking release of the inhibitory neurotransmitter glycine
- E.
Blocking the release of acetylcholine
- F.
Lysis of RBCs
- G.
Stimulation of guanylate cyclase
Correct Answer
D. Blocking release of the inhibitory neurotransmitter glycineExplanation
The correct answer is D. The penetrating wound from the rusty nail puts this patient at risk for infection with Clostridium tetani, whose symptoms are caused by the tetanus toxin. This toxin blocks the release of glycine from Renshaw cells in the spinal cord and results in “lockjaw” and other similar symptoms. Tetanus re-vaccination is required approximately every 10 years to ensure adequate blood levels of protective antibodies.
Answer A is incorrect. The Vibrio cholera toxin ADP-ribosylates a G protein in the intestine, increasing adenylate cyclase activity and causing pumping of water and chloride ions into the gut lumen. Its most characteristic symptom is voluminous “rice water” diarrhea. The heat-labile toxin of Escherichia coli has the same mechanism of action.
Answer B is incorrect. The Corynebacterium diphtheriae toxin inactivates the elongation factor EF-2 by ADP ribosylation, disrupting protein synthesis. It causes pharyngitis and “pseudomembrane” in the throat. Although the patient’s prior vaccinations most likely included vaccination for diphtheria as well, this infection is less likely than tetanus to occur with a penetrating wound.
Answer C is incorrect. Superantigens such as the Staphylococcus aureus toxin and the Streptococcus
pyogenes erythrogenic toxin bind to the MHC class II receptor and the T-lymphocyte receptor, resulting in cytokine release and sometimes toxic shock syndrome.
Answer E is incorrect. The Clostridium botulinum toxin inhibits the release of acetylcholine, resulting in anticholinergic symptoms and even CNS. C. botulinum is most often found in canned food and honey (resulting in “floppy baby” syndrome when consumed by young children).
Answer F is incorrect. The streptolysin O toxin of Streptococcus pyogenes is a hemolysin.
Answer G is incorrect. The heat-stable toxin of Escherichia coli stimulates guanylate cyclase.Rate this question:
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- 5.
A 50-year-old man develops nonbloody watery diarrhea while working as an aid worker in a refugee camp in Bangladesh. He arrived in the area 2 days ago. A stool smear shows no WBCs. He subsequently develops dehydration and electrolyte abnormalities leading to cardiac and renal failure. Which of the following organisms is the most likely cause of this man’s enterocolitis?
- A.
Clostridium diffi cile
- B.
Giardia lamblia
- C.
Helicobacter pylori
- D.
Salmonella species
- E.
Vibrio cholerae
Correct Answer
E. Vibrio choleraeExplanation
The correct answer is E. Vibrio cholerae causes watery stools, often called rice-water stool. This illness is not accompanied by abdominal pain, but the symptoms are due to dehydration leading to electrolyte imbalances. Cholera toxin causes uncontrolled stimulation of adenylate cyclase; the resulting excess of cAMP causes uncontrolled secretion of chloride and water (due to the osmotic gradient), resulting in extremely watery diarrhea accompanied by electrolyte imbalances.
Answer A is incorrect. Clostridium difficile causes severe nonbloody diarrhea associated with pseudomembranes. Diarrhea may be bloody when very severe mucosal ulceration occurs. C. difficile infection is associated with previous antibiotic treatment.
Answer B is incorrect. Giardia lamblia is a protozoan parasite that is a frequent cause of nonbloody diarrhea. The stools are usually foul-smelling and contain fat (steatorrhea). It is less severe than the diarrhea caused by Vibrio cholerae.
Answer C is incorrect. Helicobacter pylori infection causes gastritis and would not cause the
symptoms described in this patient.
Answer D is incorrect. Salmonella species invade the mucosa to cause bloody diarrhea and can be acquired from poultry, meat, and eggs.Rate this question:
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- 6.
Twenty-four hours after placement of a catheter, a hospitalized patient develops a fever and chills. Within 1 hour her systolic blood pressure falls 30 points and she develops swelling in her extremities. Despite valiant efforts by the hospital staff, the patient dies. X-ray of the patient’s lungs taken only hours before she passed away shows pulmonary edema. Which of the following mediators of this patient’s disease process is most likely responsible for the pathology described?
- A.
C3a
- B.
C5a
- C.
Hageman factor
- D.
γ-Interferon
- E.
Interleukin-1
- F.
Nitric oxide
Correct Answer
A. C3aExplanation
The correct answer is A. This patient suffered
from shock, most likely endotoxic shock due to
gram-negative bacteremia. The endotoxin lipopolysaccharide
(LPS) is found in the cell
wall of gram-negative bacteria. LPS activates
the alternative pathway of the complement cascade.
The patient’s pulmonary edema contributes
to the acute respiratory distress syndrome
that accompanies septic shock. The C3a component
of the complement cascade contributes
to the hypotension and edema seen in endotoxic
shock.
Answer B is incorrect. The C5a component of
the complement cascade, activated by endotoxin,
functions in neutrophil chemotaxis.
Answer C is incorrect. Endotoxin can directly
activate Hageman factor, activating the coagulation
cascade and leading to disseminated intravascular
coagulation.
Answer D is incorrect. γ-Interferon is produced
by T lymphocytes and, among other
functions, activates tumoricidal macrophages.
Answer E is incorrect. The cytokine interleukin-
1, released by macrophages activated by
endotoxin, causes fever.
Answer F is incorrect. Nitric oxide, released
by macrophages activated by endotoxin, causes
hypotension (shock).Rate this question:
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- 7.
A 31-year-old pregnant woman comes to the physician because of painful vesicular lesions that have recently appeared on her genitalia. A positive result on which of the following diagnostic tests would mean that her baby is at risk for congenital anomalies?
- A.
Anti-hepatitis B surface antibody test
- B.
Giemsa stain for cytoplasmic inclusions
- C.
Monospot test
- D.
Tzanck smear for multinucleated giant cells
- E.
Weil-Felix test
Correct Answer
D. Tzanck smear for multinucleated giant cellsExplanation
The correct answer is D. ToRCHeS is an acronym
for organisms that can cross the placenta
and cause congenital anomalies: Toxoplasmosis,
Rubella, Cytomegalovirus, HIV/
Herpes, and Syphilis. Genital lesions suggest a
sexually transmitted disease. The Tzanck test is
a smear of an opened skin vesicle that detects
multinucleated giant cells, indicative of HSV
types 1 and 2 or VZV. HSV-1, HSV-2, and
VZV may all be transmitted vertically to the fetus.
Remember: “Tzanck heaven I don’t have
herpes!” In the United States today, CMV is
the most common cause of congenital abnormalities.
Answer A is incorrect. The presence of anti-
HB surface antibody indicates immunity to the
hepatitis B virus, either by previous exposure or
by vaccination. While active or chronic hepatitis
B can be vertically transmitted to the fetus,
it does not cause congenital anomalies. Furthermore,
hepatitis B is not associated with
genital lesions.
Answer B is incorrect. Cytoplasmic inclusions
seen on Giemsa or fl uorescent antibodystained
smear suggest Chlamydia trachomatis.
Although Chlamydia can be vertically transmitted
to the fetus at delivery, it is not associated
with congenital anomalies or with painful
vesicular lesions. It can, however, cause blindness
and pneumonia in the newborn and
should be treated during pregnancy.
Answer C is incorrect. The monospot test detects
heterophile antibodies by the agglutination
of sheep RBCs, indicative of EBV infection.
While EBV can cause mononucleosis
and Burkitt’s lymphoma, it is not one of the
ToRCHeS organisms and does not transmit
pavertically
to the fetus. Moreover, EBV does not
produce genital lesions.
Answer E is incorrect. The Weil-Felix test uses
Proteus antigen to test for antirickettsial antibodies,
indicative of typhus or Rocky Mountain
spotted fever. Rickettsia species can cause
headache, fever, and rash; however, they do
not produce genital lesions.Rate this question:
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- 8.
A 13-year-old girl who returned a few days ago from a school camping trip in North Carolina is home ill from school. She tells her parents that she has a headache and the chills. Over the next few days, she develops a rash that begins on her palms and soles, but spreads inward to her wrists and ankles and then to her trunk. Her worsening condition leads her parents to take her to the emergency department, where a blood test reveals antibodies that react with the Proteus antigen. This patient is most likely infected with which of the following?
- A.
Borrelia burgdorferi
- B.
Coxiella burnetti
- C.
Coxsackievirus A
- D.
Rickettsia rickettsii
- E.
Rickettsia typhi
- F.
Treponema pallidum
Correct Answer
D. Rickettsia rickettsiiExplanation
The correct answer is D. This patient most
likely has Rocky Mountain spotted fever, as indicated
by the rash on her palms and soles and
the inward, “centripetal” pattern of spread.
Other supporting evidence are the accompanying
headache and fever, and a positive Weil-
Felix reaction, which is a cross-reaction of certain
anti-rickettsial antibodies with the Proteus
antigen. Rocky Mountain spotted fever is
caused by the rickettsial organism Rickettsia
rickettsii, and is endemic to the east coast of
the United States. It is transmitted by the
Ixodes tick, thus the patient probably acquired
it during her recent camping trip.
Answer A is incorrect. Borrelia burgdorferi
causes Lyme disease. The rash of Lyme disease
is typically a bull’s-eye type rash, with negative
Weil-Felix reaction.
Answer B is incorrect. Coxiella burnetti is also
a rickettsial organism. It is transmitted by aerosols,
causes Q fever, and has no associated rash
or positive Weil-Felix reaction.
Answer C is incorrect. Coxsackie A is an RNA
virus that causes hand, foot, and mouth disease,
which can also present with a rash on the
palms and soles, in addition to oral and occasionally
genital lesions. However, the positive
Weil-Felix reaction in this case and recent history
of a camping trip point to Rickettsia rickettsiae
as a more likely causative organism in
this case.
Answer E is incorrect. Rickettsia typhi causes
endemic typhus, and is transmitted by fl eas.
The rash of typhus is centrifugal; it spreads outward,
not inward as in this case.
Answer F is incorrect. Treponema pallidum is
the spirochete that causes syphilis, a sexually
transmitted disease. Although syphilis can also
present with a rash on the palms and soles, this
patient has no history of a sexual encounter
that would put her at risk for this disease.Rate this question:
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- 9.
A 53-year-old obese man with poorly controlled non-insulin-dependent diabetes mellitus presents with fever to 39.6° C (103.2° F), jaundice, hypotension, and acute onset of right upper quadrant pain. Right upper quadrant imaging shows multiple gallstones and cholecystitis. Urgent cholecystectomy is performed, and subsequent gall bladder fl uid and blood cultures grow aerobic, non-lactose-fermenting, oxidasepositive, gram-negative rods. Blood tests show: Hematocrit: 29% WBC count: 14,700/mm3 Platelet count: 76,000/mm3 D-dimer: 8500 ng/mL Fibrinogen levels: low Microscopic inspection of peripheral blood smear shows schistocytes and multiple helmet cells. Clinically, there is no evidence of active bleeding. What is the most appropriate treatment for this patient’s coagulopathy? International Normalized Ratio: 3.2
- A.
Amoxicillin
- B.
Aztreonam
- C.
Fresh frozen plasma
- D.
Vancomycin
- E.
Vitamin K
Correct Answer
B. AztreonamExplanation
The correct answer is B. This patient has leukocytosis
and Charcot’s triad (fever, jaundice,
right upper quadrant pain), along with the ominous
sign of hypotension, a clear clinical picture
of cholecystitis. In addition, he has
Pseudomonas aeruginosa sepsis and disseminated
intravascular coagulation (DIC). Gramnegative
rod sepsis is the clear cause of this patient’s
DIC, and antipseudomonal coverage
with aztreonam is most appropriate. Aztreonam
is a β-lactamase-resistant monobactam
that interferes with cell wall biosynthesis by
binding to penicillin-binding protein 3. Aztreonam
is a potent antipseudomonal agent indicated
for pseudomonal sepsis.
Answer A is incorrect. Amoxicillin is an aminopenicillin
antibiotic that interferes with cell
wall synthesis. Although amoxicillin has an extended
spectrum compared with penicillin
(covering Haemophilus infl uenzae, Escherichia
coli, Listeria, Proteus, Salmonella, and Enterococci),
it does not provide antipseudomonal
coverage.
Answer C is incorrect. Use of fresh frozen
plasma (FFP) is reserved for patients with coagulopathy
and signs of active, life-threatening
bleeding. Although provision of FFP will temporarily
reverse some of this patient’s laboratory
signs of DIC (elevated International Normalized
Ratio, decreased fi brinogen), treatment of
the underlying cause (ie, Pseudomonas sepsis)
is most important.
Answer D is incorrect. Vancomycin is an antibiotic
used for serious multidrug-resistant,
gram-positive infections. Major uses are for
methicillin-resistant Staphylococcus aureus and
moderate to severe Clostridium diffi cile infections.
Its mechanism of action is to inhibit cell
wall mucopeptide formation by binding the
D-ala-D-ala portion of cell wall precursors.
Answer E is incorrect. Coagulopathy caused
by warfarin overdose is reversed by pharmacologic
administration of vitamin K. This patient’s coagulopathy is caused by Pseudomonas
sepsis, so vitamin K therapy plays no role here.Rate this question:
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- 10.
A 22-year-old woman presents to the physician with vaginal itching and burning. On examination, she has a foul-smelling greenish discharge. A swab sample is taken and a wet mount slide is prepared; results are shown in the image. Which of the following medications should be prescribed for this patient?
- A.
Metronidazole
- B.
Nifurtimox
- C.
Quinine
- D.
Sodium stibogluconate
- E.
Sulfadiazine and pyrimethamine
Correct Answer
A. MetronidazoleExplanation
The correct answer is A. Trichomonas vaginalis
is the cause of vaginitis. Symptoms of vaginitis
include a foul-smelling greenish discharge,
itching, and burning. T. vaginalis is
transmitted sexually. On microscopic wet
mount, one fi nds trophozoites. Metronidazole
is used to treat T. vaginalis infection.
Answer B is incorrect. Nifurtimox is used to
treat Trypanosoma cruzi. T. cruzi infection
causes Chagas’ disease, a condition in which
the heart is enlarged and fl accid. T. cruzi is
transmitted via the reduviid bug. Microscopic
examination reveals fl agellated trypomastigotes
in the blood and nonmotile amastigotes in tissue
culture.
Answer C is incorrect. Quinine is used to treat
babesiosis. Babesia species present with a malaria-
like syndrome. Babesiosis is transmitted
by the Ixodes tick. On microscopic examination,
one observes no red blood cell pigment
and the Maltese cross-appearing parasite.
Answer D is incorrect. Sodium stibogluconate
is used to treat Leishmania donovani infection.
L. donovani presents with hepatomegaly and
splenomegaly, malaise, anemia, and weight
loss. L. donovani is transmitted via the sandfl y.
Microscopically, macrophages containing
amastigotes are observed.
Answer E is incorrect. Sulfadiazine and pyrimethamine
are used to treat toxoplasmosis.
Toxoplasma gondii infection presents with
brain abscesses in HIV-positive patients and
with birth defects. T. gondii is transmitted via
cysts in raw meat or cat feces. The defi nitive
stage (sexual stage) occurs in cats. Microscopically,
acid-fast staining cysts are found.Rate this question:
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- 11.
A 40-year-old man goes on a camping vacation with his family. One day after swimming in a freshwater lake near the camp site, he develops nausea and vomiting and starts to behave irrationally. His family takes him to the emergency department, where blood samples are taken and a spinal tap is performed. He is diagnosed with a rapidly progressing meningoencephalitis and dies shortly thereafter. Which of the following protozoa was most likely the cause of the man’s illness?
- A.
Cryptosporidium species
- B.
Entamoeba histolytica
- C.
Leishmania donovani
- D.
Naegleria fowleri
- E.
Plasmodium falciparum
Correct Answer
D. Naegleria fowleriExplanation
The correct answer is D. Naegleria fowleri
presents with a rapidly progressing meningoencephalitis
that can progress to coma or death
within 6 days. Other symptoms include nausea,
vomiting, and irrational behavior. Transmission
occurs through swimming in freshwater
lakes. Microscopic analysis will reveal
amebas in the spinal fl uid. Unfortunately,
there is no treatment for N. fowleri.
Answer A is incorrect. Cryptosporidium species
infection presents with severe diarrhea in
HIV-positive patients and mild watery diarrhea
in HIV-negative patients. Cryptosporidium species
are transmitted via cysts in water (fecal-oral
transmission). Microscopically, acid-fast staining
cysts are found. Unfortunately, there is no
treatment available for Cryptosporidium species
Infection; however, in healthy patients,
cryptosporidiosis is self-resolving.
Answer B is incorrect. Entamoeba histolytica
infection presents with bloody diarrhea (dysentery),
abdominal cramps with tenesmus, and
pus in the stool. It can also cause right upper
quadrant pain and liver abscesses. E. histolytica
is transmitted via cysts in water (fecal-oral transmission).
On microscopy, one observes amebas
with ingested RBCs. Treatment for E. histolytica
infection includes metronidazole and iodoquinol.
Answer C is incorrect. Leishmania donovani
infection presents with hepatomegaly and
spleno megaly, malaise, anemia, and weight
loss. L. donovani is transmitted via the sandfl y.
Microscopically, macrophages containing
amastigotes are observed. Sodium stibogluconate
is used to treat L. donovani infection.
Answer E is incorrect. The Plasmodium falciparum
parasite is responsible for causing malaria.
It is spread by the Anopheles mosquito.
Diagnosis of Plasmodium falciparum infection
is made through a blood smear.Rate this question:
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- 12.
A 54-year-old man presents to the clinic with scleral icterus, hepatosplenomegaly, ascites, and a history of episodes of jaundice over the past 3 years. He was involved in an auto accident when he was 21 years old, for which he required surgery and blood transfusions. Laboratory tests show: Aspartate aminotransferase: 734 U/L Alanine aminotransferase: 846 U/L Direct bilirubin: 0.1 mg/dL Indirect bilirubin: 7.6 mg/dL Assuming a viral etiology, which of the following is the most likely cause of this patient’s illness?
- A.
Hepatitis A
- B.
Hepatitis C
- C.
Hepatitis D
- D.
Hepatitis E
- E.
Hepatitis G
Correct Answer
B. Hepatitis CExplanation
The correct answer is B. This is a classic presentation
of chronic hepatitis C infection. It is
a common cause of post-transfusion viral hepatitis.
Hepatitis C is a blood-borne pathogen
that can ultimately cause cirrhosis of the liver.
Answer A is incorrect. Hepatitis A is transmitted
via the fecal-oral route and does not cause
chronic infection.
Answer C is incorrect. Hepatitis D is transmitted
parenterally and can cause infection only if
its host is coinfected with hepatitis B.
Answer D is incorrect. Hepatitis E is transmitted
via the fecal-oral route and does not cause
chronic infection.
Answer E is incorrect. Hepatitis G is a transmissible
fl avivirus that has not been shown to
cause liver disease.Rate this question:
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- 13.
A 19-year-old college student presents to his family physician with a 1-week history of fever, headache, and painful exudative pharyngitis. Physical examination shows signifi cant lymphadenopathy of the cervical nodes and hepatosplenomegaly. Laboratory studies show a WBC count of 15,000/mm3 with 55% lymphocytes. A heterophile antibody test is positive. Which of the following is the most likely cause of this patient’s symptoms?
- A.
Cytomegalovirus
- B.
Epstein-Barr virus
- C.
HIV infection
- D.
Streptococcus pneumoniae
- E.
Toxoplasma gondii
Correct Answer
B. Epstein-Barr virusExplanation
The correct answer is B. EBV causes infectious
mononucleosis and is a member of the
Herpesviridae family. Mononucleosis typically
presents with high fever, elevated WBC count,
painful pharyngitis, and enlarged lymph nodes.
There are two signifi cant clinical differences
between EBV mononucleosis and that of
CMV: exudative pharyngitis and cervical
lymphadenopathy are commonly seen with
EBV, but not with CMV. A positive heterophile
antibody test is specifi c for EBV infection.
Answer A is incorrect. CMV causes a mononucleosis
syndrome that is very similar to the one
caused by EBV; however, one rarely sees an exudative
pharyngitis or cervical lymphadenopathy
when CMV is the cause. CMV can also cause
pneumonia and, if transmitted congenitally,
birth defects. After infection with CMV, most
healthy individuals are asymptomatic, but
CMV can reactivate in the immunocompromised
and cause a variety of illnesses in various
organ systems.
Answer C is incorrect. HIV is the virus that
causes AIDS. AIDS is a syndrome characterized
by a decline in CD4+ cell countRate this question:
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- 14.
A 36-year-old man comes to the physician complaining of an aching back, high fever, and vomiting of dark material. He is obviously ill and states that he has felt very poorly for approximately 1 week. Physical examination shows that the patient has a temperature of 39° C (102.2° F) and icteric sclera. The patient recently returned from a trip on safari in Africa. If a liver biopsy were done, it would show the following pathology. What are the names of the eosinophilic globules shown in this image?
- A.
Councilman bodies
- B.
Döhle bodies
- C.
Negri bodies
- D.
Pappenheimer bodies
- E.
Weibel-Palade bodies
Correct Answer
A. Councilman bodiesExplanation
The correct answer is A. The disease described
is yellow fever, caused by a member of the Flaviviridae
family. It presents with symptoms of
jaundice, aching pain, and high fever. Its vector
is the mosquito. Liver biopsy can reveal
Councilman bodies, which are eosinophilic
globules believed to be the result of apoptosis
of individual hepatocytes.
Answer B is incorrect. Döhle bodies are oval
bodies found in the neutrophils of patients
with infections, trauma, pregnancy, or cancer.
Answer C is incorrect. Negri bodies are
pathognomonic for the rabies virus. They are
eosinophilic inclusion bodies found in the cytoplasm
of nerve cells of infected individuals.
Answer D is incorrect. Pappenheimer bodies
are found in RBCs in sideroblastic anemia and
sickle cell disease. They are phagosomes containing
ferruginous granules.
Answer E is incorrect. Weibel-Palade bodies
can be seen by electron microscopy in vascular
endothelial cells. They are collections of microtubules.Rate this question:
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- 15.
A 56-year-old man presents to the emergency department with sharp retrosternal pain radiating to his back and arms. The patient is sitting up and leaning forward. He states that the pain is less severe in this position and worsens when he lies down and takes a deep breath. He also indicates that he recently recovered from a fever and a cold. On physical examination a scratchy, leathery sound is heard at the lower left sternal border. An ECG is done and confi rms the diagnosis. Which of the following microorganisms is the most likely cause of this condition?
- A.
Coronavirus
- B.
Coxsackievirus
- C.
Cytomegalovirus
- D.
Epstein-Barr virus
- E.
Staphylococcus aureus
Correct Answer
B. CoxsackievirusExplanation
The correct answer is B. This patient presents
with classic signs and symptoms of pericarditis.
Pericarditis frequently follows an upper respiratory
viral infection most commonly due to coxsackie
B virus, which causes infl ammation of
the pericardial membrane. Auscultation of the
chest would reveal a pericardial friction rub
that accounts for the scratchy, leathery sound
heard during both systole and diastole. On
ECG there would be diffuse ST segment elevation
and a depression of the PR segment
unique to pericarditis.
Answer A is incorrect. Coronavirus is a common
virus that causes a self-limiting cold.
SARS-CoV, however, has been identifi ed as
the cause of severe acute respiratory syndrome
Answer C is incorrect. CMV causes a mononucleosis
syndrome in young adults similar to
that caused by EBV, which is characterized by
fever and pharyngitis. It can also cause a severe
infection in immunocompromised patients,
which is characterized by retinitis, pneumonia,
and even death.
Answer D is incorrect. EBV causes heterophilpositive
mononucleosis with symptoms of fever,
fatigue, lymphadenopathy, and lymphocytosis.
In cases of suspected EBV infection, the
peripheral blood smear should be evaluated for
atypical lymphocytes and a heterophile antibody
test should be performed.
Answer E is incorrect. Staphylococcus aureus
is a common cause of acute bacterial endocarditis
in intravenous drug users, and rarely
causes pericarditis.Rate this question:
-
- 16.
A 28-year-old man comes to the physician because of worsening muscle weakness that began in his legs and feet 3 days ago, and has now spread to his arms and hands. Other than having a fl u-like illness 2 weeks ago, the patient has been in good health. Cerebrospinal fl uid analysis shows an increased protein concentration, a normal cell count, and a normal glucose level. An infection with which of the following organisms is the most likely cause of the nervous system syndrome described in this patient?
- A.
Candida albicans
- B.
Legionella pneumophila
- C.
Mycoplasma pneumoniae
- D.
Pseudomonas aeruginosa
- E.
Streptococcus pneumoniae
Correct Answer
C. Mycoplasma pneumoniaeExplanation
The correct answer is C. The syndrome described
is Guillain-Barré syndrome, a common
cause of acute peripheral neuropathy that results
in progressive weakness over a period of
days. Although one-third of patients report no
history of an antecedent infection, the other
two-thirds have recently experienced an acute
gastrointestinal or infl uenza-like illness prior to
developing the neuropathy. The most common
epidemiologic associations involve infections
with Campylobacter jejuni, Hae mophilus infl uenzae,
CMV, EBV, Mycoplasma pneumoniae,
and VZV. Laboratory abnormalities associated
with Guillain-Barré syndrome include elevated
gamma-globulin, decreased nerve conduction
velocity indicative of demyelination, and albuminocytologic
dissociation (CSF shows increased
protein concentration with normal cell
count in the setting of normal glucose). Although
the organisms listed frequently precede
the syndrome, there has never been any consistent
demonstration of any single infectious
agent in the peripheral nerves of these patients,
and the cause of the disease is thought to be
mediated by hypersensitive T lymphocytes.
Answer A is incorrect. Although immunocompromised
patients may be at greater risk for the
organisms that are commonly associated with
Guillain-Barré syndrome, there is no indication
this patient has a weak immune system.
Furthermore, Candida albicans does not have
any association with the syndrome.
Answer B is incorrect. While Legionnaire’s
disease has been at least anecdotally associated
with Guillain-Barré syndrome, an otherwise
healthy 28-year-old man would not be expected
to develop an infection with Legionella
pneumophila.
Answer D is incorrect. Pseudomonas aeruginosa
is not an organism associated with Guillain-
Barré syndrome.
Answer E is incorrect. Pneumococcal pneumonia
is not associated with the development
of Guillain-Barré syndrome.Rate this question:
-
- 17.
A 55-year-old man comes to his physician with a tender, swollen, and erythematous left knee. He has limited range of motion in his leg. On aspiration of the synovial fl uid from his knee, the fl uid is found to be yellow and cloudy and has 150,000 neutrophils/mm3. Gram stain of the aspirate shows gram-positive cocci in clusters. The organism most likely responsible for this patient’s symptoms has which of the following properties?
- A.
Catalase-positive and coagulase-negative
- B.
Catalase-positive and coagulase-positive
- C.
Glycoprotein capsule
- D.
α-Hemolysis
- E.
Soluble in bile
Correct Answer
B. Catalase-positive and coagulase-positiveExplanation
The correct answer is B. This man is suffering
from septic arthritis, commonly characterized
by a swollen, tender, and erythematous joint.
The organism most commonly responsible for
this infection is Staphylococcus aureus. The infection
results from the invasion of the bacteria
into the synovial fl uid. The diagnosis of septic
arthritis requires aspiration of the synovial
fl uid, which appears yellow and turbid with a
predominance of neutrophils. When Staphylococcus
is the causative agent, Gram stain and
culture of the synovial fl uid show gram-positive
cocci in clusters. S. aureus is catalase-positive
and coagulase-positive.
Answer A is incorrect. While S. aureus is indeed
catalase-positive, it is not coagulase-negative.
This answer instead describes the properties
of S. epidermidis.
Answer C is incorrect. S. aureus does not possess
a glycoprotein capsule. S. pneumoniae is a
gram-positive cocci that does possess a glycoprotein
capsule.
Answer D is incorrect. S. aureus displays a β-
hemolytic pattern, not an α-hemolytic pattern.
Answer E is incorrect. Bile solubility is not a
characteristic property of S. aureus; instead, it
is a property of S. pneumoniae.Rate this question:
-
- 18.
A family who recently emigrated from Romania brings their 7-year-old child to the pediatrician with complaints of conjunctivitis and periorbital swelling. The child has had coughing with a runny nose and high fever for 3 days. Small lesions with blue-white centers are seen in his oral cavity. Which of the following is the most likely cause of this child’s symptoms?
- A.
Diphtheria
- B.
Pertussis
- C.
Roseola
- D.
Rubella
- E.
Rubeola
Correct Answer
E. RubeolaExplanation
The correct answer is E. Rubeola, also called
measles, is a relatively rare illness in the United
States because of the ubiquity of the measles/
mumps/rubella (MMR) vaccine. It presents
with the prodrome described in this patient.
The rash that spreads from head to toe over a
3-day period develops 1 or 2 days after the appearance
of Koplik’s spots, which are red oral
lesions with blue-white centers.
Answer A is incorrect. Diphtheria is an illness
virtually unknown in the United States because
of the prevalence of the DTaP vaccine.
It is caused by Corynebacterium diphtheriae
and is characterized by a membranous pharyngitis.
Answer B is incorrect. Pertussis, or whooping
cough, is also rare due to widespread vaccinations.
It is a respiratory infection of children
that characteristically produces coughing
spasms followed by a loud inspiratory whoop.
Answer C is incorrect. Roseola is a febrile disease
of very young children that begins with a
high fever and progresses to a rash similar to
measles. Infants and young children are most
at risk. It is believed to be caused by human
herpesvirus 6.
Answer D is incorrect. Rubella, also known as
German measles, is a less severe viral exanthem.
Many infections are subclinical, but rubella
can cause severe birth defects when infection
occurs during the prenatal period.Rate this question:
-
- 19.
A 34-year-old woman newly diagnosed with HIV infection is unable to access antiretroviral medications. She is aware that if untreated, her disease will progress and make her susceptible to different infections. She inquires about the natural course her disease may take and the different infections she might acquire. Which of the following conditions is most likely to manifest only when the T cell count drops below 50/mm3?
- A.
Disseminated Mycobacterium aviumintracellulare
- B.
Herpes simplex virus infection
- C.
Herpes zoster infection
- D.
Oral thrush
- E.
Pneumonia due to Pneumocystis jiroveci
- F.
Toxoplasmosis brain lesion
Correct Answer
A. Disseminated Mycobacterium aviumintracellulareExplanation
The correct answer is A. Disseminated Mycobacterium
avium-intracellulare commonly infects
birds and other animals. It can infect humans
when their T lymphocyte count is below
approximately 50/mm3. It presents as a chronic
wasting illness as the bacteria proliferate
throughout the body. Prophylaxis is with
azithromycin. This is the only answer choice
whose development requires a T lymphocyte
countRate this question:
-
- 20.
An 8-year-old boy is brought to his pediatrician by his parents because of a fever and a sore throat. On examination, he has tonsillar exudates and swollen, tender anterior cervical nodes. His parents report no history of cough. Gram stain of the tonsillar exudate reveals gram-positive cocci. Which of the following describes the organism most likely causing this patient’s symptoms?
- A.
Catalase-negative, α-hemolytic, optochinresistant
- B.
Catalase-negative, α-hemolytic, optochinsensitive
- C.
Catalase-negative, β-hemolytic, bacitracinresistant
- D.
Catalase-negative, β-hemolytic, bacitracinsensitive
- E.
Catalase-positive, coagulase-negative, novobiocin- resistant
- F.
Catalase-positive, coagulase-negative, novobiocin- sensitive
Correct Answer
D. Catalase-negative, β-hemolytic, bacitracinsensitiveExplanation
The correct answer is D. This patient has a
classic presentation (fever, sore throat, anterior
cervical lymphadenopathy, lack of cough) of
streptococcal pharyngitis, or strep throat. Strep
throat is caused by Streptococcus pyogenes, or
group A streptococcus. Diagnosis is confi rmed
with a rapid strep test or with throat swab culture.
The treatment of choice is penicillin. To
differentiate this organism from other gram-positive organisms, several tests can be performed
in the lab. For example, when cultured
on blood agar, S. pyogenes creates a clear halo
(β-hemolysis) around the colonies due to damage
of the RBCs. To further differentiate streptococcal
species, sensitivity to different antibiotics
is measured. S. pyogenes is sensitive to
bacitracin.
Answer A is incorrect. Catalase-negative,
α-hemolytic, optochin-resistant describes the
viridans streptococci. Streptococcus mutans is
associated with the formation of dental caries.
Answer B is incorrect. Catalase-negative,
α-hemolytic, optochin-sensitive describes Streptococcus
pneumoniae. This organism causes
pneumonia and otitis media. Rates of S. pneumoniae
meningitis have decreased with the advent
of the pneumococcal vaccine.
Answer C is incorrect. Catalase-negative,
β-hemolytic, bacitracin-resistant describes Streptococcus
agalactiae, or group B streptococcus. S.
agalactiae is a signifi cant cause of serious bacterial
infection in neonates.
Answer E is incorrect. Catalase-positive, coagulase-
negative, novobiocin-resistant describes
Staphylococcus saprophyticus. This is the second
most common cause of urinary tract infection
in young, healthy women.
Answer F is incorrect. Catalase-positive, coagulase-
negative, novobiocin-sensitive describes
Staphylococcus epidermidis. Infection with S.
epidermidis is associated with skin penetration
by implanted prosthetic devices such as prosthetic
heart valves, intravenous lines, and intraperitoneal
catheters.Rate this question:
-
.JPG "A 32-year-old man presents to his doctor with painful urination and a purulent urethral discharge. The image shows cells that have been cultured from this discharge. Which of the following is the treatment of choice for this infection?")
.JPG "A 22-year-old woman presents to the physician
with vaginal itching and burning. On examination,
she has a foul-smelling greenish discharge.
A swab sample is taken and a wet
mount slide is prepared; results are shown in
the image. Which of the following medications
should be prescribed for this patient?")
.JPG "A 36-year-old man comes to the physician
complaining of an aching back, high fever, and
vomiting of dark material. He is obviously ill
and states that he has felt very poorly for approximately
1 week. Physical examination
shows that the patient has a temperature of
39° C (102.2° F) and icteric sclera. The patient
recently returned from a trip on safari in
Africa. If a liver biopsy were done, it would
show the following pathology. What are the
names of the eosinophilic globules shown in
this image?")
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Mar 20, 2023Quiz Edited by
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