A 16-year-old boy has short stature, round face, obesity, and brachydactyly with short fourth metacarpals. Serum calcium is 7.4 mg/dL, phosphate 5.8 mg/dL, and intact PTH 340 pg/mL (normal 15 to 65). Vitamin D levels are adequate. Subcutaneous ossifications are noted over his knuckles. Which defect explains these findings?

• A End-organ resistance to PTH due to Gsα mutation ✓
• B Deficient PTH secretion by the parathyroid glands
• C Activating mutation of the calcium sensing receptor
• D Vitamin D receptor mutation causing rickets

Explanation

Albright hereditary osteodystrophy features with hypocalcaemia, hyperphosphataemia and markedly elevated PTH define pseudohypoparathyroidism type 1a, caused by maternal GNAS inactivation producing renal resistance to PTH. True hypoparathyroidism would show low or inappropriately normal PTH. Activating CASR mutations cause autosomal dominant hypocalcaemia with low PTH. Hereditary vitamin D resistant rickets presents with rachitic bone changes and secondary hyperparathyroidism but not the phenotypic osteodystrophy described here.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.