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

What is a key characteristic of Pseudohypoparathyroidism type 1A (Albright hereditary osteodystrophy)?

1. Autosomal recessive inheritance pattern

2. Unresponsiveness of kidney to PTH despite low PTH levels

3. Typically presents with tall stature

4. Caused by Gs protein alpha-subunit hyperactivity

The correct answer is: Unresponsiveness of kidney to PTH despite low PTH levels

Pseudohypoparathyroidism type 1A, also known as Albright hereditary osteodystrophy, is primarily characterized by the body's unresponsiveness to parathyroid hormone (PTH) despite the presence of elevated PTH levels in the bloodstream. This lack of responsiveness occurs due to a defect in the G protein signaling pathway that transmits the PTH signal in target tissues, particularly in the kidneys and bones. As a result, patients present with symptoms of hypocalcemia and hyperphosphatemia, but they have high levels of PTH circulating due to this resistance. This condition is associated with specific physical features, such as short stature and brachydactyly, rather than tall stature, as seen in other conditions. The inheritance pattern is not autosomal recessive; instead, it typically follows an imprinting pattern affecting the paternal allele, leading to the distinct symptoms. Hyperactivity of the Gs protein alpha subunit does not characterize this disorder; instead, in Pseudohypoparathyroidism type 1A, there is a defect or resistance related to the Gs protein signaling. Understanding the mechanisms behind this condition is critical for healthcare professionals. It exemplifies how hormonal signaling can be disrupted at the level