**Core Concept**
The changing character of a murmur in a patient with joint pain and embolic phenomenon is indicative of a condition where the body's immune response is directed against its own tissues, leading to inflammation and damage. This condition is characterized by the deposition of immune complexes in various tissues, including the joints and heart.

**Why the Correct Answer is Right**
The correct answer is related to the concept of Libman-Sacks endocarditis, a form of non-bacterial endocarditis associated with systemic lupus erythematosus (SLE). In SLE, the immune complexes deposited in the heart valves can lead to valve damage and the development of a new or changing murmur. The presence of joint pain (arthralgia) and embolic phenomenon (e.g., stroke or pulmonary embolism) are also common manifestations of SLE. The changing character of the murmur is due to the intermittent nature of the valve damage caused by the immune complexes.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not related to the changing character of a murmur in the context of joint pain and embolic phenomenon.
**Option B:** This option is not a condition that is associated with the changing character of a murmur in the context of joint pain and embolic phenomenon.
**Option C:** This option is not a condition that is associated with the changing character of a murmur in the context of joint pain and embolic phenomenon.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the changing character of a murmur can be a sign of a systemic disease process, such as SLE, and not just a benign cardiac anomaly.

**Correct Answer:** C. Libman-Sacks endocarditis