**Core Concept**
Rheumatic heart disease (RHD) is a complication of rheumatic fever, where the immune system attacks the heart valves, often leading to chronic inflammation and scarring. In this context, the patient's symptoms of fever, hematuria, and palpitations may indicate a cardiac complication.

**Why the Correct Answer is Right**
The correct diagnosis is Libman-Sacks endocarditis, a rare condition associated with systemic lupus erythematosus (SLE) and other autoimmune diseases. In RHD patients, the increased risk of developing Libman-Sacks endocarditis is due to the chronic inflammation and valve damage. The condition is characterized by the formation of sterile vegetations on the heart valves, which can lead to valve dysfunction and complications such as fever, hematuria, and palpitations.

**Why Each Wrong Option is Incorrect**
* **Option A:** Acute rheumatic fever relapse is unlikely in a 2-year-old known case, as the condition is typically self-limiting and resolves within a few weeks.
* **Option B:** Infective endocarditis is a possible complication of RHD, but the presence of Libman-Sacks endocarditis is more likely given the patient's autoimmune background and symptoms.
* **Option C:** Cardiac myxoma is a type of benign tumor, but it is not typically associated with the symptoms and underlying conditions described.

**Clinical Pearl / High-Yield Fact**
Libman-Sacks endocarditis is a rare but serious complication of RHD and autoimmune diseases, highlighting the importance of considering a patient's overall medical history when evaluating cardiac symptoms.

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