USMLE Step 1
Microbiology


Q1
A 30-year old sexually active man presents with a painful vesicle on his external genitalia and
bilateral inguinal lymphadenopathy. A Tznack smear from the vesicle fails is negative, and
polymerase chain reaction analysis of viral DNA is negative. A VDRL assay is also negative. Which
of the following medications would be most helpful to this patient?
A, Acyclovir
B, Erythromycin
C, Foscarnet
D, Ribavirin
E, Vancomycin

Q1
A 30-year old sexually active woman presents with a painful vesicle on her external genitalia and
bilateral inguinal lymphadenopathy. A Tznack smear from the vesicle is negative, and PCR analysis
of viral DNA is negative. A VDRL assay is also negative. Which of the following medications would
be most helpful to this patient?
A, Acyclovir
B, Erythromycin
C, Foscarnet
D, Ribavirin
E, Vancomycin

Q1
Differential diagnosis
1.Primary syphilis
2.Gentital herpes
3.Chancroid

Medical Microbiology, Murray et al., p. 350



Q1
Primary syphilis
Painless ulcer with raised borders 10 to 90 days after initial infection
Secondary syphilis
Flu-like symptoms with a generalized mucocutaneous rash
Tertiary syphilis
Diffuse, chronic inflammation affecting various organs (e.g. neurosyphilis, cardiovascular
syphilis) accompanied by gummas

Medical Microbiology, Murray et al., p. 353



Medical Microbiology, Murray et al., p. 464



Medical Microbiology, Murray et al., p. 468



Q1
Tznack smear
A scraping of a base of a lesion
Cowdry type A acidophilic intranuclear inclusion bodies
A result of HSV disease mechanism
Inhibition of cellular macromolecular synthesis
Degradation of host cell DNA
Membrane permeation
Cytoskeletal disruption
Senescence of the cell

Q1
Chancroid
STD
More often symptomatic in men
Tender papule with erythematous base on the gentitalia or perianal area (5 - 7 days after
infection)
Painful ulcer after 2 days, inguinal lymphadenopathy
Exclude syphilis and HSV
Microbe: Haemophilus ducreyi

Q1
Microbe: Haemophilus ducreyi
Microscopy
Small G (-) rod
Culture
Gonococcal agar
1-2% hemoglobin
5% fetal bovine serum
IsoViteleX (enrichment for fastidious organisms)
Vancomycin 3 ug/ml
Biochemical tests

Q1
A 30-year old sexually active woman presents with a painful vesicle on her external genitalia and
bilateral inguinal lymphadenopathy. A Tznack smear from the vesicle is negative, and PCR analysis
of viral DNA is negative. A VDRL assay is also negative. Which of the following medications would
be most helpful to this patient?
A, Acyclovir
B, Erythromycin
C, Foscarnet
D, Ribavirin
E, Vancomycin

Q1
Acyclovir
Inhibits herpes viral polymerase (HSV 1,2; VSV; EBV)
Foscarnet
Inhibits viral DNA polymerase (CMV; acyclovir-resistant HSV)
Ribavirin
Inhibits inosine monophosphate dehydrogenase (RSV)
Vancomycin
G (+) multidrug-resistant organisms (e.g. S. aureus, C. difficile)

Q1
Microbe: Haemophilus ducreyi
Treatment
Most isolates of H. ducreyi are susceptible to ERYTHROMYCIN (drug of choice)

Q1
A 30-year old sexually active woman presents with a painful vesicle on her external genitalia and
bilateral inguinal lymphadenopathy. A Tznack smear from the vesicle is negative, and PCR analysis
of viral DNA is negative. A VDRL assay is also negative. Which of the following medications would
be most helpful to this patient?
A, Acyclovir
B, Erythromycin
C, Foscarnet
D, Ribavirin
E, Vancomycin

Sources
First Aid Q&A for the USMLE STEP 1 (2nd edition)
Medical Microbiology, Murray et al.

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