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

Which condition results in unresponsiveness of the kidney to PTH, leading to hypocalcemia despite elevated PTH levels?

1. Jod-Basedow phenomenon

2. Nelson Synd

3. Pseudohypoparathyroidism type 1A (Albright hereditary osteodystrophy)

4. Pheochromocytoma

The correct answer is: Pseudohypoparathyroidism type 1A (Albright hereditary osteodystrophy)

The condition that results in unresponsiveness of the kidney to parathyroid hormone (PTH), leading to hypocalcemia despite elevated PTH levels, is Pseudohypoparathyroidism type 1A, also known as Albright hereditary osteodystrophy. In this disorder, there is a genetic defect affecting the G protein signaling pathway, which leads to a decreased response of target tissues, including the kidneys, to PTH. As a result, although PTH levels are elevated (due to the low calcium levels triggering the parathyroid glands), the kidneys do not adequately respond to PTH, causing continued renal loss of calcium and failure to mobilize calcium from bone. This lack of responsiveness results in low serum calcium levels (hypocalcemia) and can also lead to related clinical manifestations, such as the skeletal deformations often associated with this condition. In contrast, the other conditions listed do not cause this specific unresponsiveness of the kidney to PTH. For instance, the Jod-Basedow phenomenon relates to hyperthyroidism due to iodine excess, while Nelson's syndrome is characterized by pituitary adenoma due to loss of feedback inhibition from adrenal hormones. Pheochromocytoma, which involves catecholamine