A 37-year-old female presents to the emergency room with a fever. Chest x-ray shows multiple patchy infiltrates in both lungs. Echocardiography and blood cultures suggest a diagnosis of acute bacterial endocarditis limited to the tricuspid valve. Which of the following is the most probable etiology?

**Core Concept**
Acute bacterial endocarditis (ABE) is a life-threatening infection of the heart valves, often caused by intravenous drug use or other predisposing conditions. The tricuspid valve is the most commonly affected valve in ABE, especially in individuals with a history of intravenous drug use.

**Why the Correct Answer is Right**
Intravenous drug users are at high risk of developing ABE due to the introduction of bacteria into the bloodstream through contaminated needles. The most common cause of ABE in this population is _Staphylococcus aureus_, particularly methicillin-resistant _S. aureus_ (MRSA). This bacterium can colonize the tricuspid valve and cause inflammation, leading to the characteristic patchy infiltrates seen on chest x-ray. The presence of fever and positive blood cultures further supports this diagnosis.

**Why Each Wrong Option is Incorrect**
* **Option A:** While _Streptococcus viridans_ is a common cause of subacute bacterial endocarditis, it is less likely to cause ABE, especially in the context of intravenous drug use.
* **Option B:** _Enterococcus faecalis_ can cause endocarditis, but it is more commonly associated with urinary tract infections and less commonly with ABE.
* **Option D:** _Candida albicans_ is a fungus that can cause endocarditis, particularly in patients with prosthetic valves or other predisposing conditions. However, it is less likely to be the primary cause of ABE in this scenario.

**Clinical Pearl / High-Yield Fact**
Intravenous drug users with ABE are at high risk of developing complications such as septic pulmonary emboli, heart failure, and embolic events. Prompt recognition and treatment of ABE are essential to prevent these complications and improve outcomes.

**Correct Answer:** C.

✓ Correct Answer: B. Illicit drug use