What is the major etiologic lesion in primary hyperparathyroidism?

• A. Single parathyroid adenoma
• B. Parathyroid carcinoma
• C. Multigland hyperplasia of all four glands
• D. Ectopic PTH production

Answer: (A)

In primary hyperparathyroidism, the most common cause is a solitary parathyroid adenoma—a single gland becomes autonomous and overproduces PTH, leading to inappropriately high PTH levels and hypercalcemia. This clonal adenoma accounts for the majority of cases, making it the best answer.

Parathyroid carcinoma is rare, and multigland hyperplasia involving all four glands occurs less frequently than a solitary adenoma. Ectopic PTH production is not the typical driver of primary disease; it would represent an unusual and uncommon mechanism rather than the major etiologic lesion.