**Core Concept**
The patient's symptoms are suggestive of systemic lupus erythematosus (SLE), a chronic autoimmune disease characterized by inflammation and damage to various organs, including the heart, kidneys, skin, and joints. The presence of ANA and anti-double-stranded DNA antibodies is diagnostic of SLE.

**Why the Correct Answer is Right**
The heart murmur in this patient is likely caused by Libman-Sacks endocarditis, a form of nonbacterial endocarditis that occurs in patients with SLE. Libman-Sacks endocarditis is characterized by the formation of sterile vegetations on the heart valves, which can lead to valve dysfunction and heart failure. The inflammation and immune complex deposition in SLE can lead to the development of Libman-Sacks endocarditis.

**Why Each Wrong Option is Incorrect**
**Option A:** Mitral valve prolapse is a condition characterized by the bulging of the mitral valve into the left atrium during systole. While it can cause a heart murmur, it is not directly related to SLE or the patient's symptoms.

**Option B:** Rheumatic heart disease is a complication of rheumatic fever, which is caused by streptococcal infection. The patient's blood cultures are negative, making rheumatic heart disease an unlikely diagnosis.

**Option C:** Tricuspid regurgitation is a condition characterized by the backflow of blood from the right ventricle to the right atrium during systole. While it can cause a heart murmur, it is not directly related to SLE or the patient's symptoms.

**Option D:** Aortic stenosis is a condition characterized by the narrowing of the aortic valve opening, which can lead to a heart murmur. However, it is not directly related to SLE or the patient's symptoms.

**Clinical Pearl / High-Yield Fact**
Libman-Sacks endocarditis is a classic example of a "nonbacterial" endocarditis, which is often seen in patients with autoimmune diseases such as SLE. It is essential to consider this diagnosis in patients with SLE who present with heart murmurs.

**Correct Answer:** C.