**Core Concept**
Subacute bacterial endocarditis (SBE) is a type of infective endocarditis that develops over weeks to months, characterized by the colonization of bacteria on pre-existing heart valves, leading to valve damage and potential embolic and systemic complications.

**Why the Correct Answer is Right**
SBE is most commonly caused by **Streptococcus viridans**, a group of alpha-hemolytic bacteria that are part of the normal oral flora. These bacteria can enter the bloodstream and colonize damaged heart valves, leading to the development of SBE. The mechanism involves the adherence of bacteria to the valve surface, followed by the formation of a biofilm and the subsequent activation of the host's immune response.

**Why Each Wrong Option is Incorrect**
* **Option A:** Staphylococcus aureus is more commonly associated with acute bacterial endocarditis, which has a more rapid onset and is often caused by direct inoculation of bacteria into the bloodstream through a skin or mucous membrane injury.
* **Option B:** Enterococcus faecalis can cause endocarditis, but it is more commonly seen in patients with underlying medical conditions such as urinary tract infections or prosthetic valves.
* **Option D:** Echinococcus granulosus is a parasite that causes hydatid disease, which can involve the heart, but it is not a common cause of bacterial endocarditis.

**Clinical Pearl / High-Yield Fact**
The modified Duke criteria are a useful tool for diagnosing infective endocarditis, which includes major criteria such as positive blood cultures, evidence of valve damage, and embolic events, as well as minor criteria such as fever, weight loss, and heart murmur.

**Correct Answer:** C. Streptococcus viridans.