Classic features of VDDR type 2?

• A. Vitamin D receptor resistance with rickets and alopecia.
• B. Vitamin D deficiency with alopecia.
• C. Hyperparathyroidism with osteitis fibrosa.
• D. Hypophosphatemic rickets with normal hair.

Answer: (C)

Vitamin D receptor resistance causes end-organ insensitivity to the active vitamin D hormone, so tissues don’t respond to 1,25-dihydroxyvitamin D despite its presence. This leads to rickets or osteomalacia from impaired mineralization, and a striking feature is alopecia because hair follicles also rely on VDR signaling. In VDDR type 2, you often see secondary hyperparathyroidism due to hypocalcemia, with inappropriately high or normal levels of 1,25-dihydroxyvitamin D given the receptor defect. The combination of rickets with alopecia is the hallmark, which is why the option describing vitamin D receptor resistance with rickets and alopecia best fits VDDR type 2. The other descriptions describe vitamin D deficiency, hyperparathyroidism-related bone disease, or hypophosphatemic rickets—none of which capture this receptor-resistant form with alopecia.