What is a key distinguishing feature of vitamin D-dependent rickets type II (VDDR-II)?

• A. End-organ resistance to calcitriol due to VDR mutation, often with alopecia
• B. Excess calcitriol production
• C. Hypophosphatemia due to renal phosphate wasting
• D. Low PTH independent of calcium

Answer: (A)

End-organ resistance to calcitriol due to mutations in the vitamin D receptor is the defining feature of this condition. Because the receptor is defective, tissues such as the intestine and bone cannot respond to 1,25-dihydroxyvitamin D even when its levels are normal or elevated. This impaired signaling leads to poor calcium and phosphate absorption and defective bone mineralization, which drives rickets. Alopecia is a notable clue because the vitamin D receptor also plays a role in hair follicle biology, so its dysfunction often manifests with hair loss.

That’s why this option best captures the essence: the problem is not overproduction of calcitriol, nor a primary phosphate-wasting process, and PTH dynamics relate to the resulting hypocalcemia rather than being independent of calcium.