A 52-year-old male presents with generalized weakness, recurrent kidney stones, and bone pain. Blood tests show serum calcium 11.8 mg/dL (reference: 8.5-10.2), serum phosphate 2.1 mg/dL (reference: 2.5-4.5), and inappropriately elevated intact parathyroid hormone (iPTH). What is the most common cause of primary hyperparathyroidism?

Answer and explanation

Correct answer: A. Solitary benign parathyroid adenoma

  1. Step 1: Interpret biochemical profile: Hypercalcemia paired with hypophosphatemia and elevated/inappropriately normal PTH confirms primary hyperparathyroidism.
  2. Step 2: Recall epidemiological etiology: A solitary benign adenoma accounts for approximately 85% to 90% of all primary hyperparathyroidism cases.
  3. Step 3: Conclude diagnosis: Solitary benign parathyroid adenoma.
Answer
Solitary benign parathyroid adenoma.
Why not the others: 'Four-gland hyperplasia' accounts for 10-15% and is often associated with MEN syndromes; 'Parathyroid carcinoma' is extremely rare (<1%); 'Ectopic PTHrP' causes humoral hypercalcemia of malignancy with suppressed intact PTH.

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