**Core Concept**
The patient's vegetations are sterile, non-destructive, and located on the mitral valve leaflets, suggesting a non-infectious etiology. This presentation is characteristic of a condition caused by an abnormal coagulation pathway.

**Why the Correct Answer is Right**
The patient's vegetations are most likely due to Libman-Sacks endocarditis, which is a form of non-bacterial thrombotic endocarditis (NBTE) associated with systemic lupus erythematosus (SLE). This condition is caused by the deposition of immune complexes and fibrin on the valve leaflets, leading to sterile vegetations. The pathogenesis involves the activation of the coagulation pathway, leading to the formation of thrombi on the valve surface.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because Libman-Sacks endocarditis is a non-infectious form of endocarditis, whereas infectious endocarditis typically presents with destructive vegetations and fever. Infectious endocarditis is more common in patients with pre-existing valve disease or intravenous drug use.
* **Option B:** This option is incorrect because Marantic endocarditis, also known as non-bacterial thrombotic endocarditis (NBTE), typically occurs in the setting of malignancy or hypercoagulable states. While it can present with sterile vegetations, it is less commonly associated with SLE.
* **Option C:** This option is incorrect because rheumatic heart disease typically presents with valvular damage and scarring, rather than sterile vegetations.

**Clinical Pearl / High-Yield Fact**
Libman-Sacks endocarditis is a classic association with SLE, and its presence can be a clue to the underlying diagnosis in patients with unexplained valvular abnormalities.

**Correct Answer:** B.