## Core Concept
Mercury poisoning primarily affects the kidneys, and within the renal tubules, it has a specific site of action. The renal tubules are crucial for the reabsorption and secretion of substances, and heavy metals like mercury can cause damage to these tubules.

## Why the Correct Answer is Right
Mercury is known to cause damage to the **proximal convoluted tubule (PCT)** of the renal tubules. The PCT is responsible for the reabsorption of glucose, amino acids, and other valuable nutrients back into the bloodstream. Mercury ions can bind to the thiol groups of enzymes and proteins in the PCT cells, leading to cellular damage and necrosis. This is because the PCT has a high concentration of thiol groups, making it particularly susceptible to mercury toxicity.

## Why Each Wrong Option is Incorrect
* **Option A:** This option is incorrect because mercury primarily affects the proximal parts of the renal tubule, not the distal parts.
* **Option B:** This option is incorrect because, although the loop of Henle is crucial for concentrating urine, it is not the primary site of mercury toxicity.
* **Option D:** This option is incorrect because the collecting duct, while important for final urine concentration and electrolyte balance, is not the primary site of action for mercury toxicity.

## Clinical Pearl / High-Yield Fact
A key point to remember is that heavy metal poisoning, including mercury, can lead to **acute kidney injury (AKI)**. The damage to the PCT can impair the kidney's ability to reabsorb nutrients and can lead to the presence of glucose, amino acids, and other substances in the urine. A classic clinical correlation is that mercury poisoning can cause **proteinuria** and **glycosuria** due to PCT damage.

## Correct Answer: C. Proximal convoluted tubule.