**Core Concept**
Renal tuberculosis is an infectious disease caused by Mycobacterium tuberculosis that primarily affects the kidneys. Early detection of renal tuberculosis is crucial for effective treatment and prevention of kidney damage. Imaging modalities play a vital role in diagnosing renal tuberculosis.

**Why the Correct Answer is Right**
The most sensitive imaging modality to detect early renal tuberculosis is ultrasound, specifically the **intrarenal echogenic foci** and **hydro-ureteroenoscopy** findings. Ultrasound is non-invasive, cost-effective, and readily available. It can detect early changes such as renal cortical thinning, caliectasis, and perinephric fat stranding, which are indicative of renal tuberculosis.

**Why Each Wrong Option is Incorrect**
* **Option A:** CT scan with contrast is highly sensitive for detecting renal tuberculosis, but it is not the most sensitive modality for early detection. CT scan may miss early changes and is more useful for identifying complications such as calcifications and abscesses.
* **Option B:** MRI is not the most sensitive modality for detecting early renal tuberculosis. While it can provide detailed images of the kidneys, it is not as readily available as ultrasound and is more expensive.
* **Option C:** X-ray is not a sensitive modality for detecting renal tuberculosis. It may show calcifications in advanced cases, but it is not useful for early detection.

**Clinical Pearl / High-Yield Fact**
Renal tuberculosis often presents with non-specific symptoms such as flank pain, hematuria, and pyuria. It is essential to maintain a high index of suspicion for renal tuberculosis, especially in patients with a history of tuberculosis exposure or in regions with high prevalence.

**Correct Answer: D. Ultrasound.**