**Core Concept**
The investigation of choice for studying a renal cortical mass involves a radiopharmaceutical that selectively binds to the renal cortex, providing information on renal function and cortical morphology. This requires a radiopharmaceutical that is taken up by the renal cortex and retained in the kidneys, allowing for the assessment of cortical thickness, scarring, and other abnormalities.

**Why the Correct Answer is Right**
99mTc-DMSA (Dimercaptosuccinic acid) is a radiopharmaceutical that is specifically taken up by the renal cortical tissue, providing high-resolution images of the renal cortex. This is due to the fact that DMSA has a high affinity for the proximal convoluted tubules and the renal cortex, where it is retained for an extended period. This allows for the assessment of renal cortical morphology, including cortical thickness, scarring, and other abnormalities, making it an ideal agent for the evaluation of renal cortical masses.

**Why Each Wrong Option is Incorrect**
**Option A:** 99mTc-DTPA (Diethylenetriamine pentaacetic acid) is a radiopharmaceutical that is primarily excreted by glomerular filtration and is not retained in the renal cortex, making it less suitable for the evaluation of cortical masses.

**Option B:** 51Cr-EDTA (Ethylenediaminetetraacetic acid) is a radiopharmaceutical that is used to assess glomerular filtration rate (GFR) and is not specifically taken up by the renal cortex.

**Option D:** 99mTc-Pyrophosphate is a radiopharmaceutical that is used to assess myocardial damage and is not specifically taken up by the renal cortex.

**Clinical Pearl / High-Yield Fact**
DMSA scans are particularly useful in the evaluation of renal scarring and cortical damage, often in the context of pyelonephritis or other renal infections. This is because DMSA binds to the renal cortex, allowing for the assessment of cortical morphology and scarring.

**✓ Correct Answer: C. 99 Tc DMSA**