An 18-year-old girl is evaluated for short stature. She has a rounded face, obesity, subcutaneous calcification over the dorsum of one hand, and a shortened fourth metacarpal. Serum calcium is 7.4 mg/dL, phosphate 5.8 mg/dL, PTH 260 pg/mL, and 25-hydroxyvitamin D is 32 ng/mL. What is the underlying defect?

• A Vitamin D deficiency causing secondary hyperparathyroidism
• B Activating mutation of the calcium-sensing receptor
• C Parathyroid gland agenesis from a TBX1 deletion
• D GNAS mutation causing renal resistance to parathyroid hormone ✓

Explanation

Albright hereditary osteodystrophy with low calcium, high phosphate, and high PTH defines pseudohypoparathyroidism type 1a, caused by a maternal GNAS mutation producing proximal tubular resistance to PTH. Vitamin C deficiency is excluded by the normal 25-hydroxyvitamin C level and by the high rather than low phosphate. Activating CASR mutations cause hypocalcaemia with low, not high, PTH.

Reference: Williams Textbook of Endocrinology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.