**Core Concept**
Infective endocarditis (IE) in patients with rheumatic heart disease (RHD) is a serious complication due to impaired valve function and chronic inflammation. Vegetations are focal, fibrin-rich masses on heart valves that form due to bacterial or fungal colonization. In RHD, the valvular damage creates a favorable environment for microbial adhesion, and the composition of vegetations varies by pathogen.

**Why the Correct Answer is Right**
In patients with pre-existing rheumatic heart disease, the valves are structurally damaged and prone to infection. While *Streptococcus viridans* (A) and *Staphylococcus aureus* (B) are common causes of IE, *Candida albicans* (C) is increasingly recognized as a major cause of vegetation formation in patients with chronic valvular disease. In such cases, fungal vegetations are often large, friable, and can be seen on echocardiography. *Salmonella typhi* (D) is not a common cause of endocarditis and is typically associated with typhoid fever, not vegetations on rheumatic valves.

**Why Each Wrong Option is Incorrect**
Option A: *Streptococcus viridans* is a common cause of IE in immunocompetent individuals, especially in dental infections, but it typically forms smaller vegetations and is less likely to produce the largest vegetations in rheumatic heart disease.
Option B: *Staphylococcus aureus* causes severe, often life-threatening IE, but it is more common in intravenous drug users or those with prosthetic valves, not typically in rheumatic heart disease.
Option D: *Salmonella typhi* causes typhoid fever and may rarely lead to IE, but it does not form large vegetations in the context of rheumatic heart disease and is not a leading pathogen in such cases.

**Clinical Pearl / High-Yield Fact**
In patients with rheumatic heart disease, fungal endocarditis—especially *Candida albicans*—is a significant and underrecognized cause of large, complex vegetations, particularly in the setting of long-standing valve damage and immunosuppression.

✓ Correct Answer: C. Candida albicans