Where are vitamin D receptors (VDR) located in the body?

• A. Parathyroid glands
• B. Muscle tissue
• C. Cardiovascular system
• D. Kidneys

Answer: (D)

Vitamin D works by binding to a receptor that acts in the cell nucleus to regulate gene expression. This receptor is present in many tissues, which is why vitamin D can influence calcium and phosphate balance in multiple places. The kidneys are a primary target because they express the receptor and play a central role in calcium and phosphate handling; activation of the vitamin D pathway there promotes calcium reabsorption and phosphate regulation and helps modulate PTH activity. While other tissues like parathyroid, muscle, and even cardiovascular tissues can have the receptor, the kidneys are a classic, well-established site of vitamin D receptor activity among the options.