**Core Concept**
The synthesis of metabolically active vitamin D involves a complex biochemical pathway that involves the conversion of cholesterol to calcitriol (1,25-dihydroxyvitamin D3) in the skin, liver, and kidneys.

**Why the Correct Answer is Right**
Vitamin D synthesis begins in the skin where 7-dehydrocholesterol is exposed to ultraviolet B (UVB) radiation from sunlight, resulting in the formation of cholecalciferol (vitamin D3). This compound is then transported to the liver where it undergoes hydroxylation to form 25-hydroxyvitamin D (25(OH)D). Finally, in the kidneys, 25(OH)D is converted to calcitriol (1,25-dihydroxyvitamin D3) through the action of the enzyme 1-alpha-hydroxylase. Calcitriol is the biologically active form of vitamin D that plays a crucial role in maintaining calcium and phosphate homeostasis.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it does not describe the vitamin D synthesis pathway accurately. While it is true that vitamin D3 is produced in the skin, it does not mention the subsequent steps in the pathway.
**Option B:** This option is incorrect as it only mentions the production of vitamin D3 in the skin, without describing the conversion to calcitriol in the kidneys.
**Option C:** This option is incorrect as it does not accurately describe the vitamin D synthesis pathway. While it mentions the conversion of 25(OH)D to calcitriol, it does not specify the enzyme responsible for this conversion.

**Clinical Pearl / High-Yield Fact**
It's essential to note that vitamin D deficiency is a common problem worldwide, particularly in regions with limited sunlight exposure. Calcitriol plays a crucial role in maintaining bone health, and its deficiency can lead to conditions such as rickets in children and osteomalacia in adults.

**Correct Answer:** This question does not provide enough information to determine the correct answer.