**Core Concept**
Bacterial endocarditis is a serious infection of the heart valves, typically caused by bacteria entering the bloodstream and colonizing the valve. This condition often affects individuals with pre-existing heart conditions, such as valvular disease or congenital heart defects.

**Why the Correct Answer is Right**
Streptococci, specifically viridans streptococci, are the most common cause of bacterial endocarditis. These bacteria are part of the normal oral flora and can enter the bloodstream through dental procedures, gum disease, or other mucosal injuries. The viridans streptococci adhere to the damaged valve endothelium, leading to the formation of biofilms and subsequent infection. The bacteria then trigger an immune response, resulting in inflammation and damage to the valve.

**Why Each Wrong Option is Incorrect**
**Option A:** Staphylococcus aureus is a common cause of bacterial endocarditis, particularly in intravenous drug users and those with prosthetic valves. However, it is not the most common cause.
**Option B:** Enterococci are another type of bacteria that can cause endocarditis, especially in hospitalized patients with urinary tract infections or other underlying conditions. However, they are less common than viridans streptococci.
**Option C:** Candida is a fungus that can cause endocarditis, particularly in patients with compromised immune systems or prosthetic valves. However, fungal endocarditis is much less common than bacterial endocarditis.

**Clinical Pearl / High-Yield Fact**
To reduce the risk of bacterial endocarditis, patients with pre-existing heart conditions should receive prophylactic antibiotics before undergoing dental procedures involving the gums or peridental tissues. This is especially important for patients with prosthetic valves or a history of rheumatic fever.

**Correct Answer: C. Streptococci**