**Core Concept**
The renal scarring defect in the kidney can be investigated using various imaging modalities. The investigation of choice should provide detailed anatomical information about the kidney, including the presence of scars, cysts, or any other abnormalities.

**Why the Correct Answer is Right**
The correct investigation of choice for renal scarring defect is DMSA (Dimercaptosuccinic Acid) scan. This nuclear medicine scan involves the administration of a radioactive compound that binds to the renal cortex, allowing for the visualization of renal parenchymal abnormalities, including scars. The DMSA scan is particularly useful in detecting cortical scarring and is often used in the evaluation of patients with recurrent urinary tract infections, vesicoureteral reflux, or other conditions that may lead to renal scarring.

**Why Each Wrong Option is Incorrect**
**Option A:** Ultrasonography is a useful imaging modality for evaluating renal anatomy and detecting obstructive uropathy, but it is not the investigation of choice for renal scarring defect.
**Option B:** CT scan is a valuable tool for detecting renal calculi, tumors, and other abnormalities, but it is not typically used for evaluating renal scarring.
**Option C:** MRI is a sensitive imaging modality for detecting renal abnormalities, including scarring, but it is not the first-line investigation of choice for renal scarring defect.

**Clinical Pearl / High-Yield Fact**
The DMSA scan is particularly useful in detecting cortical scarring in the early stages, before it progresses to more severe renal damage. It is essential to perform a DMSA scan within 2-3 months after a urinary tract infection to accurately diagnose renal scarring.

**Correct Answer:** D. DMSA scan