**Core Concept**
1,25-Dihydroxyvitamin D (1,25(OH)2D), also known as calcitriol, is the hormonally active form of vitamin D3. It plays a crucial role in regulating calcium and phosphate homeostasis in the body.

**Why the Correct Answer is Right**
1,25(OH)2D is synthesized in the kidneys through a two-step process involving the enzyme 1-alpha-hydroxylase, which converts 25-hydroxyvitamin D (25(OH)D) into 1,25(OH)2D. This process is tightly regulated by parathyroid hormone (PTH), which stimulates the production of 1,25(OH)2D in response to low calcium levels. The resulting calcitriol then acts on the intestines, bone, and kidneys to maintain normal calcium and phosphate levels in the body.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because 1,25(OH)2D is not primarily formed in the liver, where it is synthesized from vitamin D3. While the liver does play a role in the initial hydroxylation of vitamin D3, the final step of 1-alpha-hydroxylation occurs in the kidneys.
* **Option B:** This option is incorrect because the skin is responsible for the initial synthesis of vitamin D3 from cholesterol, but not the conversion of 25(OH)D to 1,25(OH)2D, which occurs in the kidneys.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the kidneys regulate 1,25(OH)2D production in response to PTH levels, and renal impairment can lead to decreased calcitriol production, contributing to secondary hyperparathyroidism and osteomalacia.

**Correct Answer: C. Kidneys**