Final answer:
The most common organism causing endocarditis in patients with abnormal cardiac valves is Streptococcus viridans, especially in those with conditions such as congenital heart disease or rheumatic heart disease. Option c.
Step-by-step explanation:
The most common organism that causes endocarditis with abnormal cardiac valves—such as those associated with congenital heart disease, rheumatic heart disease, or postoperative changes—is Streptococcus viridans. This bacterium is part of the normal flora of the mouth but can enter the bloodstream during events that cause breaches in the body's natural defenses, such as dental procedures or surgery.
Endocarditis due to S. viridans often takes the subacute form, where the damage to the heart valves occurs slowly over time, and it is particularly problematic in individuals with preexisting heart conditions. The formation of vegetations and possible fibrosis of heart valves due to the immune response can severely damage the heart and may require treatment with high doses of intravenous antibiotics.