**Core Concept**
Mercury toxicity primarily affects the kidneys by damaging the renal cortex, particularly the proximal convoluted tubules. This nephrotoxicity is due to the accumulation of mercury in the renal tissue, leading to cellular damage and dysfunction.

**Why the Correct Answer is Right**
Mercury, especially mercuric ions, accumulate in the proximal convoluted tubules of the kidneys, where they can cause oxidative stress, mitochondrial damage, and eventually, necrosis of renal tubular cells. This results in impaired renal function, including decreased glomerular filtration rate and increased serum creatinine levels. The proximal convoluted tubules are particularly susceptible to mercury toxicity due to their high concentration of sulfhydryl groups, which can bind to mercury ions.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because the distal convoluted tubules are not the primary site of mercury toxicity, although they may also be affected.
**Option B:** Incorrect because the glomerulus is not directly affected by mercury toxicity, although overall renal function may be impaired.
**Option C:** Incorrect because the renal medulla is not the primary site of mercury toxicity, although it may be affected indirectly due to impaired renal function.

**Clinical Pearl / High-Yield Fact**
Mercury toxicity can also cause nephrotic syndrome, characterized by massive proteinuria, hypoalbuminemia, and hyperlipidemia. Early recognition and treatment of mercury exposure are essential to prevent long-term renal damage.

**Correct Answer:** C.