A Gram-negative bacterium injects bacterial proteins directly into host cells using a specialized needle-like appendage to manipulate host cellular processes. This virulence mechanism is characteristic of which system?

The correct answer to the question "A Gram-negative bacterium injects bacterial proteins directly into host cells using a specialized needle-like appendage to manipulate host cellular processes. This virulence mechanism is characteristic of which system?" is Option C: Type III secretion system.

The other options — A) Type I secretion system, B) Type II secretion system, D) Type IV secretion system — are incorrect for this USMLE Exam Prep question. As explained above: The Type III secretion system (T3SS) is often called an 'injectisome.' It directly injects effector proteins from the bacterial cytoplasm into the host cell cytoplasm, bypassing the extracellular env…