USMLE (United States Medical Licensing Examination) Step 1 Practice Exam

What is the most common cause of primary hyperparathyroidism?

1. Increased PTH levels

2. Decreased serum phosphorus

3. Decreased serum calcium

4. Benign adenoma

The correct answer is: Benign adenoma

Primary hyperparathyroidism is most commonly caused by a benign adenoma, which is a non-cancerous tumor of the parathyroid gland. This tumor leads to the overproduction of parathyroid hormone (PTH), resulting in increased calcium levels in the blood (hypercalcemia). The benign adenoma typically arises from one of the parathyroid glands, causing an imbalance in calcium homeostasis by promoting excessive bone resorption, renal tubular reabsorption of calcium, and increased intestinal absorption of calcium. The role of elevated PTH levels is a consequence of the adenoma; hence it is not the cause but rather the effect of the tumor's activity. Decreased serum phosphorus is often observed in hyperparathyroidism due to PTH's action on the kidneys, but the decreased phosphorus levels arise from the disease rather than causing it. Lastly, decreased serum calcium would not be associated with primary hyperparathyroidism, as the hallmark of this condition is actually elevated serum calcium due to the overactivity of the parathyroid glands. Understanding the pathophysiology of primary hyperparathyroidism emphasizes the significance of the benign adenoma as the underlying cause, making it the most common reason for this condition.