**Core Concept**
Renal tuberculosis is a form of extrapulmonary tuberculosis that affects the kidneys, typically resulting from the hematogenous spread of Mycobacterium tuberculosis from a primary pulmonary focus. This condition is characterized by the destruction of renal tissue, leading to various clinical manifestations.

**Why the Correct Answer is Right**
Sterile pyuria, which is the presence of white blood cells in the urine without the presence of bacteria, is a hallmark of renal tuberculosis. This occurs due to the activation of the immune system in response to the tuberculous bacilli. The infection triggers an inflammatory response, resulting in the release of pro-inflammatory cytokines and the recruitment of leukocytes to the affected renal tissue. The subsequent destruction of renal tissue leads to the presence of white blood cells in the urine, despite the absence of bacterial growth.

**Why Each Wrong Option is Incorrect**
**Option A:** Loin pain is a common symptom of renal tuberculosis, but it is not the most characteristic feature of the disease. While loin pain can occur, it is not as specific as sterile pyuria for diagnosing renal tuberculosis.

**Option B:** Painful hematuria is not a typical presentation of renal tuberculosis. Hematuria can occur in advanced cases, but it is usually painless and not a primary symptom of the disease.

**Option D:** Constitutional symptoms, such as fever, weight loss, and fatigue, can occur in patients with renal tuberculosis, but they are non-specific and can be seen in various other conditions. These symptoms are not unique to renal tuberculosis and do not distinguish it from other forms of tuberculosis or other diseases.

**Clinical Pearl / High-Yield Fact**
The presence of sterile pyuria in a patient with symptoms suggestive of renal disease should raise the suspicion of renal tuberculosis. A thorough investigation, including imaging studies and microbiological examination, should be performed to confirm the diagnosis.

✓ Correct Answer: C. Sterile pyuria