**Core Concept**
1α-hydroxylation is a crucial enzymatic step in vitamin D metabolism that converts 25-hydroxyvitamin D to its active form, 1,25-dihydroxyvitamin D (calcitriol). This activation is essential for calcium homeostasis and bone health.

**Why the Correct Answer is Right**
The enzyme 1α-hydroxylase, primarily located in the **proximal convoluted tubule cells of the kidney**, catalyzes the addition of a hydroxyl group at the 1-position of the vitamin D molecule. This step transforms 25-hydroxyvitamin D (calcifediol) into 1,25-dihydroxyvitamin D (calcitriol), the biologically active form that binds to vitamin D receptors in target tissues. This process is tightly regulated by parathyroid hormone (PTH) and serum calcium levels.

**Why Each Wrong Option is Incorrect**
Option A: Skin is where 7-dehydroxylation occurs, converting 7-dehydrocholesterol to previtamin D3, not 1α-hydroxylation.
Option B: The liver performs 25-hydroxylation, converting vitamin D3 to 25-hydroxyvitamin D, not 1α-hydroxylation.
Option D: Blood does not contain the enzyme responsible for 1α-hydroxylation; it only transports the vitamin D metabolites.

**Clinical Pearl / High-Yield Fact**
The kidney is the **only organ** capable of 1α-hydroxylation. In renal failure, this process is impaired, leading to secondary hyperparathyroidism and hypocalcemia — a key reason for calcium and vitamin D supplementation in dialysis patients.

✓ Correct Answer: C. Kidney