**Core Concept**
Rheumatic heart disease can lead to complications such as Libman-Sacks endocarditis, which involves the formation of sterile vegetations on the heart valves. In children, this can be associated with systemic symptoms like fever, splenomegaly, and microscopic hematuria due to the deposition of immune complexes.

**Why the Correct Answer is Right**
The presence of low-grade fever, splenomegaly, clubbing, and microscopic hematuria in a child with a history of rheumatic heart disease suggests the possibility of Libman-Sacks endocarditis. The most important diagnostic test in this scenario is an **echocardiogram** to visualize the heart valves and detect any vegetations or valvular dysfunction. This is because echocardiography is highly sensitive for detecting valvular abnormalities and endocarditis, especially in children.

**Why Each Wrong Option is Incorrect**
* **Option A:** Renal biopsy would be more relevant for diagnosing glomerulonephritis, which is not the primary concern in this case.
* **Option B:** Chest X-ray might show cardiomegaly or pulmonary edema but would not provide specific information about the valvular disease.
* **Option D:** Complete blood count (CBC) might show anemia or leukocytosis but would not aid in the diagnosis of Libman-Sacks endocarditis.

**Clinical Pearl / High-Yield Fact**
In children with a history of rheumatic heart disease, the presence of systemic symptoms like fever and splenomegaly, along with microscopic hematuria, should raise suspicion for Libman-Sacks endocarditis. Echocardiography is the key diagnostic test for this condition.

**Correct Answer:** C. Echocardiogram