**Core Concept**
The question highlights the risk of infective endocarditis following a dental procedure in a patient with rheumatic heart disease (RHD). This condition is often caused by bacteria that colonize the oral cavity. The patient's compromised heart valve increases the likelihood of bacterial seeding and subsequent infection.

**Why the Correct Answer is Right**
Streptococcus viridans is a group of viridans streptococci, which are commonly found in the oral cavity. These bacteria are known to be responsible for infective endocarditis, particularly in patients with pre-existing heart conditions. The bacteria can enter the bloodstream during dental procedures, such as extractions, and then adhere to the damaged heart valve, leading to infection. The viridans streptococci can produce biofilms on the valve surface, making them resistant to antibiotics and host immune responses.

**Why Each Wrong Option is Incorrect**
**Option B:** Streptococcus pneumoniae is a common cause of pneumonia and meningitis, but it is not typically associated with infective endocarditis following dental procedures.
**Option C:** Streptococcus pyogenes is a Group A beta-hemolytic streptococcus that causes pharyngitis and skin infections, but it is not commonly linked to infective endocarditis in this context.
**Option D:** Staphylococcus aureus is a common cause of skin and soft tissue infections, as well as endocarditis, but it is more commonly associated with healthcare-associated infections or intravenous drug use.

**Clinical Pearl / High-Yield Fact**
The American Heart Association recommends antibiotic prophylaxis for patients with high-risk heart conditions, such as RHD, before dental procedures that involve manipulation of the gingival tissue or the periapical region of teeth, to prevent infective endocarditis.

**✓ Correct Answer: A. Streptococcus viridans**