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

Which condition is due to a defect in the Gs protein alpha-subunit, causing end-organ resistance to PTH and characteristic physical features like shortened 4th/5th digits?

1. Jod-Basedow phenomenon

2. Nelson Synd

3. Pseudohypoparathyroidism type 1A (Albright hereditary osteodystrophy)

4. Whipple triad

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

The condition that results from a defect in the Gs protein alpha-subunit, leading to end-organ resistance to parathyroid hormone (PTH) and is characterized by physical features such as shortened 4th and 5th digits, is pseudohypoparathyroidism type 1A, also known as Albright hereditary osteodystrophy. This genetic disorder is often associated with a mutation affecting the alpha-subunit of the Gs protein, which impairs the signaling pathways for hormones like PTH. Consequently, despite elevated levels of circulating PTH, the target tissues (such as the kidneys and bones) do not respond appropriately, leading to symptoms of hypocalcemia and hyperphosphatemia. Among the characteristic physical features, individuals with this condition may display heterotopic bone formation, obesity, and specific dysmorphic features, including the classic brachydactyly, which manifests as shortened 4th and 5th digits. This association with Gs protein dysfunction underlines the unique genetic and phenotypic characteristics of the disorder. In contrast, other conditions listed do not share this specific pathophysiology. The Jod-Basedow phenomenon relates to hyperthyroidism secondary to iodine excess; Nelson syndrome usually refers