A 58-year-old man with weight loss and a lung mass develops serum calcium of 13.5 mg/dL, phosphate 1.8 mg/dL, undetectable PTH, and no skeletal metastases on bone scan. Which mediator best explains his hypercalcemia?
- A Parathyroid hormone-related peptide secreted by the tumor ✓
- B Calcitriol produced by tumor macrophages
- C Osteoclast activating factor released by plasma cells
- D Ectopic PTH secretion from the lung tumor
Correct answer: A. Parathyroid hormone-related peptide secreted by the tumor
Explanation
Humoral hypercalcemia of malignancy in squamous cell carcinoma is mediated by PTH-related peptide, which binds the PTH receptor in kidney and bone, raising calcium and lowering phosphate while native PTH is suppressed. Ectopic true PTH secretion by non-parathyroid tumors is exceedingly rare. Calcitriol-mediated hypercalcemia is typical of lymphoma, and osteoclast activating factors apply to multiple myeloma, not a solid lung mass without lytic lesions.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.