Okay, let me try to work through this question. The question is about mercury poisoning affecting a specific part of the renal tubule. Hmm, I remember that heavy metals like mercury are nephrotoxic, right? So, the kidney is a common target. But which part exactly?

Mercury is known to accumulate in the proximal tubule. Why there? Because the proximal tubule is responsible for reabsorbing a lot of filtered substances, and mercury might bind to sulfhydryl groups in proteins here. That could damage the cells. Wait, the proximal convoluted tubule is the site where most reabsorption happens, including glucose, amino acids, and ions. Mercury toxicity might lead to Fanconi syndrome, which is a dysfunction of the proximal tubule. So, the answer should be the proximal convoluted tubule.

Let me check the options. The correct answer is C, which is Proximal convoluted tubule. The other options might be distal convoluted tubule, loop of Henle, or collecting duct. Distal tubule is more involved in potassium and calcium regulation, so mercury isn't typically there. Loop of Henle is for concentrating urine, maybe affected by other toxins. Collecting duct is for water reabsorption and ADH action. So, the key here is the proximal part. Also, clinical pearl: Mercury poisoning can cause Fanconi syndrome (proximal tubule), while lead might affect the distal. So, the answer is C.

**Core Concept**
Mercury toxicity primarily targets the **proximal convoluted tubule** of the renal nephron due to its high metabolic activity and abundant sulfhydryl groups, which mercury binds to, causing cellular damage and impaired reabsorption.

**Why the Correct Answer is Right**
Mercury (especially inorganic forms like mercuric chloride) is concentrated in the proximal tubule because this segment reabsorbs ~60-70% of filtered water, sodium, and bicarbonate. Mercury binds to **thiol groups** in enzymes (e.g., Na⁺/K⁺-ATPase), disrupting ATP production and causing necrosis. This leads to **Fanconi syndrome** (generalized proximal tubule dysfunction), characterized by glycosuria, aminoaciduria, and phosphaturia. The proximal tubule’s high oxygen demand exacerbates mercury-induced oxidative stress, worsening injury.

**Why Each Wrong Option is Incorrect**
**Option A: Distal convoluted tubule** – Mercury does not preferentially accumulate here; this segment is more vulnerable to **lead** toxicity.
**Option B: Loop of Henle** – Mercury’s nephrotoxic effects are not localized to the loop; this area is more affected by **cisplatin** or **NSAIDs**.
**Option D: Collecting duct** – Mercury does not target this region; collecting duct dysfunction is seen in **lithium toxicity** or **SIADH**.

**Clinical Pearl / High-Yield Fact**
Mercury poisoning mimics Fanconi syndrome (proximal tubule injury), while lead affects the **distal convoluted tubule** (Fanconi-like syndrome but with preserved proximal function). Remember: **“Mercury in front, lead in back”** for proximal vs. distal

✓ Correct Answer: A. PCT