MD Infectious Disease and Microbiology 3

Question 1

A 7-year-old unvaccinated child develops a barking cough, stridor, and low-grade fever. Lateral neck X-ray shows a 'steeple sign.' What is the causative organism?

Accepted Answer

Parainfluenza virus

Answer

Croup (laryngotracheobronchitis) is most commonly caused by parainfluenza virus and produces subglottic narrowing, visible as the steeple sign on AP neck X-ray.

Question 2

A 55-year-old farmer develops a painless black eschar on his forearm after handling imported animal hides. Gram stain of exudate shows large gram-positive rods in chains. Which toxin mediates systemic lethality?

Accepted Answer

Protective antigen + lethal factor (lethal toxin)

Answer

Bacillus anthracis cutaneous anthrax is caused by its AB-type toxin; protective antigen (PA) binds host cells and delivers lethal factor (a metalloprotease) or edema factor, with lethal toxin being the primary cause of death.

Question 3

A neonate develops purulent conjunctivitis within 2-4 days of birth. Gram stain of discharge shows gram-negative diplococci inside neutrophils. Which prophylactic agent should have been administered at birth?

Accepted Answer

Erythromycin ophthalmic ointment

Answer

Neonatal gonococcal ophthalmia (ophthalmia neonatorum) from N. gonorrhoeae is prevented by erythromycin ophthalmic ointment, the current US standard prophylaxis.

Question 4

A 25-year-old sexually active woman has a painless genital ulcer with a raised, indurated edge and non-tender inguinal lymphadenopathy. Dark-field microscopy shows motile spirochetes. Which stage of syphilis is this?

Accepted Answer

Primary syphilis

Answer

Primary syphilis presents as a single painless, indurated chancre at the inoculation site with regional lymphadenopathy, caused by Treponema pallidum visible on dark-field microscopy.

Question 5

A transplant patient on tacrolimus develops fever and a lung nodule with surrounding halo on CT. Bronchoalveolar lavage shows septate hyphae with acute angle branching (45°). Which antifungal is first-line?

Accepted Answer

Voriconazole

Answer

Invasive pulmonary aspergillosis in immunocompromised patients is caused by Aspergillus species (septate hyphae, 45° branching) and treated first-line with voriconazole.

MD - Doctor of Medicine Practice Test

MD - Doctor of Medicine Practice Test

MD Infectious Disease and Microbiology 3