A 58-year-old woman with squamous cell carcinoma of the lung develops serum calcium of 13.5 mg/dL, phosphate of 2.0 mg/dL, undetectable PTH, and elevated nephrogenous cAMP excretion. The tumor product responsible for her findings acts by:

• A Stimulating osteoclasts directly through the RANKL-independent calcitonin receptor
• B Binding the PTH1 receptor because of N-terminal structural homology with PTH ✓
• C Inhibiting the calcium-sensing receptor on parathyroid chief cells
• D Acting as an agonist at the FGF23 receptor in proximal tubules

Explanation

PTH-related peptide shares strong N-terminal sequence homology with PTH, allowing it to bind and activate the same PTH1 receptor in bone and kidney. This reproduces the full biochemical picture of primary hyperparathyroidism except that native PTH is suppressed by hypercalcemia. It does not require the calcitonin receptor or FGF23 signalling, and the parathyroid glands are bypassed entirely.

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.