USMLE Microbiology & Immunology 2

Question 1

A sputum culture grows gram-positive cocci in clusters that are catalase-positive and coagulase-positive. What organism is this?

Accepted Answer

Staphylococcus aureus

Answer

Coagulase-positive Staphylococcus aureus is distinguished from other staphylococci by its production of coagulase, which converts fibrinogen to fibrin.

Question 2

Which type of T helper cell is responsible for orchestrating the response against intracellular pathogens by activating macrophages through IFN-gamma?

Accepted Answer

Th1

Answer

Th1 cells produce IFN-gamma and TNF-beta, activating macrophages to destroy intracellular pathogens and driving cell-mediated immunity.

Question 3

A child is vaccinated with a live attenuated vaccine. Why is this vaccine contraindicated in immunocompromised patients?

Accepted Answer

The attenuated virus can revert to virulence and cause disease in immunocompromised hosts

Answer

Live attenuated vaccines can cause disease in immunocompromised patients because the attenuated organism may revert to virulence when the immune system cannot control even weakened pathogens.

Question 4

A CSF sample shows gram-negative diplococci. What is the most likely causative organism in an 18-year-old college student?

Accepted Answer

Neisseria meningitidis

Answer

Neisseria meningitidis is the most common cause of bacterial meningitis in adolescents and young adults, particularly in close-contact settings like college dormitories.

Question 5

Which mechanism allows bacteria to acquire antibiotic resistance from the environment by taking up naked DNA?

Accepted Answer

Transformation

Answer

Transformation is the uptake of naked DNA from the environment by naturally competent bacteria, allowing acquisition of resistance genes from dead bacteria.

USMLE Practice Test

USMLE Practice Test

USMLE Microbiology & Immunology 2