**Core Concept**
Mercury is a heavy metal that can cause nephrotoxicity, particularly affecting the proximal convoluted tubule (PCT) of the kidney. This is due to its ability to accumulate in the lysosomes of renal tubular cells, leading to cellular damage and dysfunction.

**Why the Correct Answer is Right**
The PCT is the most susceptible part of the kidney to mercury toxicity, as it is responsible for the reabsorption of various substances, including heavy metals. Mercury can bind to sulfhydryl groups in the PCT, leading to the formation of mercuric sulfide, which is then engulfed by lysosomes, causing cellular damage and subsequent impairment of renal function. The PCT's high metabolic rate and the presence of lysosomes make it particularly vulnerable 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 more resistant to heavy metal accumulation due to its lower metabolic rate and lack of lysosomes.

**Option C:** The collecting duct is not directly affected by mercury toxicity, as it is primarily responsible for the regulation of electrolyte and water balance, rather than the reabsorption of heavy metals.

**Option D:** The loop of Henle is not the primary site of mercury toxicity, as it is involved in the concentration and dilution of urine, rather than the reabsorption of heavy metals.

**Clinical Pearl / High-Yield Fact**
Mercury toxicity can be prevented or minimized by using chelating agents, such as succimer or penicillamine, which can bind to mercury and facilitate its excretion from the body.

**✓ Correct Answer: A. PCT**