**Core Concept**
Mercury is a nephrotoxic heavy metal that primarily affects the proximal convoluted tubule (PCT) of the kidney, leading to acute tubular necrosis and renal failure. The PCT is responsible for reabsorbing glucose, amino acids, and other essential nutrients from the filtrate.

**Why the Correct Answer is Right**
Mercury's nephrotoxicity is primarily due to its ability to bind to sulfhydryl groups on enzymes in the PCT, leading to their inactivation and subsequent cellular damage. This results in the release of intracellular contents into the tubular lumen, causing tubular obstruction and further renal injury. The PCT's high metabolic rate and abundance of sulfhydryl-containing enzymes make it particularly susceptible to mercury toxicity.

**Why Each Wrong Option is Incorrect**
**Option B:** The distal convoluted tubule (DCT) is not primarily affected by mercury toxicity, as it is less metabolically active and has fewer sulfhydryl-containing enzymes compared to the PCT.

**Option C:** The collecting duct is responsible for regulating electrolyte and water balance, and is not directly affected by mercury toxicity.

**Option D:** The loop of Henle is involved in the concentration and dilution of urine, and is not the primary site of mercury-induced nephrotoxicity.

**Clinical Pearl / High-Yield Fact**
Mercury toxicity can be prevented by using personal protective equipment (PPE) and following proper laboratory protocols when handling heavy metals. It is essential to recognize the signs and symptoms of mercury poisoning, including renal failure, gastrointestinal symptoms, and neurological abnormalities.

**✓ Correct Answer: A. PCT**