**Question:** Which of the following is not true of Vit D?

A. Vitamin D is essential for calcium homeostasis and bone health.
B. Vitamin D is synthesized in the skin upon exposure to ultraviolet B radiation.
C. Vitamin D is involved in the immune system modulation.
D. Vitamin D deficiency is common in developed countries.

**Correct Answer: A.**

**Core Concept:** Vitamin D is a fat-soluble vitamin that plays a crucial role in calcium homeostasis and bone health. It regulates calcium and phosphate levels in the blood, ensuring proper mineralization of bones and teeth.

**Why the Correct Answer is Right:** Vitamin D has two main forms: vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). They are synthesized through two main mechanisms:

1. Ergocalciferol (Vitamin D2) is produced in plants upon exposure to ultraviolet B (UVB) radiation. Humans can obtain it through ingestion of plant sources or supplementation.
2. Cholecalciferol (Vitamin D3) is synthesized in the skin upon exposure to ultraviolet B radiation (UVB) from sunlight. Humans obtain it through sunlight exposure and dietary sources.

**Why Each Wrong Option is Incorrect:**

**A. False:** Vitamin D is not solely responsible for calcium homeostasis. While it plays a significant role, other factors like parathyroid hormone (PTH), calcium, and phosphate also contribute to calcium homeostasis.

**B. True:** Vitamin D is synthesized in the skin from cholesterol upon exposure to ultraviolet B radiation (UVB) from sunlight. This option is accurate.

**C. True:** Vitamin D modulates the immune system, which helps explain its role in autoimmune diseases and infections. This option is true.

**D. True:** Vitamin D deficiency is common in developed countries due to indoor lifestyles, limited sunlight exposure, and dietary restrictions. This option is true but not directly related to the question's focus on calcium homeostasis and bone health.

**Clinical Pearl:** Vitamin D supplementation can lead to hypercalcemia and hypercalciuria if consumed in excess, potentially causing kidney stones, hypercalcemia, and other complications. Therefore, it is essential to monitor vitamin D levels and ensure appropriate supplementation under medical supervision.