A 62-year-old male smoker presents with weight loss and confusion. Serum calcium is 13.8 mg/dL with a suppressed PTH. Chest CT shows a centrally located cavitating mass abutting the left main bronchus. Biopsy shows nests of cells with intercellular bridges and keratinization. The humoral mediator of his hypercalcemia is:
- A Osteoclast activating factor from bone metastases
- B Calcitriol produced by 1-alpha hydroxylase activity
- C Parathyroid hormone related peptide (PTHrP) ✓
- D Ectopic parathyroid hormone secretion
Correct answer: C. Parathyroid hormone related peptide (PTHrP)
Explanation
The biopsy features (keratinization, intercellular bridges) identify squamous cell carcinoma, the lung cancer classically linked to hypercalcemia via PTHrP secretion. PTHrP mimics PTH at the kidney and bone but is not measured by standard PTH assays, hence suppressed intact PTH. Calcitriol-mediated hypercalcemia occurs in lymphoma and granulomatous disease. Ectopic true PTH secretion is vanishingly rare.
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.