**Question:** Leucocidin is produced by: September 2004

**Core Concept:** Leucocidin is a group of bactericidal proteins produced by some strains of Gram-positive bacteria, most notably Staphylococcus aureus. These proteins target and kill host immune cells, particularly neutrophils and monocytes, contributing to the pathogenesis of infections caused by these bacteria.

**Why the Correct Answer is Right:** Leucocidin is produced by Staphylococcus aureus, which is a well-known human pathogen causing various infections. The correct answer (D) refers to the bacterium producing leucocidin, which aligns with our understanding of the question's context. Leucocidin is a virulence factor that contributes to the pathogenicity of S. aureus, enabling the bacteria to evade the host immune response and establish infections.

**Why Each Wrong Option is Incorrect:**

A. Staphylococcus epidermidis is another Gram-positive bacterium but does not produce leucocidin. This option is incorrect because it refers to a different bacterium that does not produce leucocidin.

B. Staphylococcus epidermidis is mentioned in option A as incorrect, but option B repeats the same information, making it incorrect as well.

C. Staphylococcus epidermidis is incorrect due to the reasons mentioned above for option A and B.

**Clinical Pearl:** Understanding leucocidin production by Staphylococcus aureus is essential for recognizing the pathogenicity and virulence factors of this bacterium. This knowledge helps in diagnosing infections caused by S. aureus and selecting appropriate antibiotics and treatment strategies.

**Correct Answer:** D. Staphylococcus epidermidis does not produce leucocidin.