**Core Concept**
1,25 Hydroxylation of Vitamin D is a crucial step in the activation of Vitamin D in the body. This process involves the conversion of 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D, which is the biologically active form of Vitamin D. The kidneys play a central role in this process through an enzyme-mediated reaction.

**Why the Correct Answer is Right**
The site of 1,25 hydroxylation of Vitamin D in the kidneys is the **Endoplasmic Reticulum (ER)** of the proximal tubule cells. This reaction is catalyzed by the enzyme **1-alpha-hydroxylase**, which is a cytochrome P450 enzyme. The 1-alpha-hydroxylase enzyme is present in the ER of the proximal tubule cells, where it converts 25-hydroxyvitamin D into 1,25-dihydroxyvitamin D. This active form of Vitamin D then plays a crucial role in maintaining calcium homeostasis and bone health.

**Why Each Wrong Option is Incorrect**
* **Option A:** Not applicable.
* **Option B:** Not correct, as the site of 1,25 hydroxylation is not in the peritubular capillaries.
* **Option D:** Not correct, as the site of 1,25 hydroxylation is not in the collecting ducts.

**Clinical Pearl / High-Yield Fact**
One important clinical correlation is that patients with chronic kidney disease (CKD) often have impaired 1,25 hydroxylation of Vitamin D, leading to secondary hyperparathyroidism and bone disease.

**Correct Answer: C. Endoplasmic Reticulum (ER) of the proximal tubule cells.**