**Question:** A 25 year old man with a 3-week fever presented with tricuspid valve vegetation. The patient is an intravenous drug abuser. Most common cause of endocarditis in this patient is -

A. Streptococcus pyogenes
B. Streptococcus pneumoniae
C. Staphylococcus aureus
D. Escherichia coli

**Correct Answer:** C. Staphylococcus aureus

**Core Concept:**

Intravenous drug abuse is a well-known risk factor for the development of infective endocarditis (IE), a life-threatening condition characterized by the formation of an infection in the heart valves or endocardial surfaces. In this scenario, the patient presents with tricuspid valve vegetation, which is a characteristic feature of IE. The answer focuses on the most common cause of IE among intravenous drug users.

**Why the Correct Answer is Right:**

Staphylococcus aureus is the most common cause of infective endocarditis in intravenous drug users. This bacterium is often part of the skin flora and can easily enter the bloodstream due to the damaged skin caused by the practice of intravenous drug use. Once in the bloodstream, Staphylococcus aureus can colonize and infect the heart valves, leading to the development of IE.

**Why Each Wrong Option is Incorrect:**

A) Streptococcus pyogenes is a less common cause of IE among intravenous drug users. It is also known as group A beta-hemolytic streptococci and is associated with other conditions like streptococcal toxic shock syndrome and scarlet fever.

B) Streptococcus pneumoniae is a common cause of community-acquired pneumonia but is less likely to cause IE in intravenous drug users due to its preference for the respiratory system.

D) Escherichia coli (E. coli) is a common cause of urinary tract infections and gastrointestinal infections, but is less likely to cause IE due to its preference for these specific systems rather than cardiac involvement.

**Clinical Pearl:**

In intravenous drug users, infective endocarditis should be suspected when presenting with fever, cardiac symptoms, and a history of intravenous drug use. Early diagnosis and initiation of appropriate antibiotics are crucial to prevent complications and improve patient outcomes.