MD Infectious Disease and Microbiology 3 — Questions and Answers
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?
- Haemophilus influenzae type b
- Bordetella pertussis
- Parainfluenza virus (Correct answer)
- Streptococcus pyogenes
Correct answer: Parainfluenza virus
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?
- Alpha toxin (lecithinase)
- Protective antigen + lethal factor (lethal toxin) (Correct answer)
- Exfoliative toxin
- Pyrogenic exotoxin A
Correct answer: Protective antigen + lethal factor (lethal toxin)
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?
- Acyclovir eye drops
- Tetracycline ointment
- Erythromycin ophthalmic ointment (Correct answer)
- Silver nitrate drops
Correct answer: Erythromycin ophthalmic ointment
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?
- Secondary syphilis
- Tertiary (late) syphilis
- Primary syphilis (Correct answer)
- Latent syphilis
Correct answer: Primary syphilis
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?
- Fluconazole
- Amphotericin B deoxycholate
- Voriconazole (Correct answer)
- Caspofungin
Correct answer: Voriconazole
Invasive pulmonary aspergillosis in immunocompromised patients is caused by Aspergillus species (septate hyphae, 45° branching) and treated first-line with voriconazole.
Question 6: A 40-year-old returns from sub-Saharan Africa with fever, painless facial swelling, and a rapidly moving subconjunctival worm. Which organism causes this?
- Wuchereria bancrofti
- Loa loa (Correct answer)
- Onchocerca volvulus
- Dracunculus medinensis
Correct answer: Loa loa
Loa loa (African eye worm) is transmitted by Chrysops flies and characteristically migrates across the conjunctiva, causing transient Calabar swellings; it is endemic to West and Central Africa.
Question 7: A 3-year-old develops fever, rash starting on the face and spreading centrifugally, and Koplik spots on the buccal mucosa. The rash is maculopapular and begins behind the ears. Which virus is responsible?
- Rubella virus
- Parvovirus B19
- Measles (Rubeola) virus (Correct answer)
- Human herpesvirus 6
Correct answer: Measles (Rubeola) virus
Measles (rubeola) classically presents with the 3 Cs (cough, coryza, conjunctivitis), Koplik spots, and a maculopapular rash spreading from head to toe.
MD Infectious Disease and Microbiology 3
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?