Pseudohypoparathyroidism is characterized by:
**Core Concept**
Pseudohypoparathyroidism is a rare genetic disorder characterized by resistance to parathyroid hormone (PTH), leading to hypocalcemia and hyperphosphatemia. This condition is caused by mutations in the GNAS gene, which encodes the Gαs subunit of G-proteins involved in PTH signaling.
**Why the Correct Answer is Right**
The correct answer involves understanding the pathophysiology of pseudohypoparathyroidism. In this condition, the GNAS gene mutation leads to a decrease in the activity of Gαs, resulting in impaired PTH signaling. Specifically, the mutant Gαs subunit fails to stimulate adenylate cyclase, which is essential for increasing intracellular cAMP levels and activating protein kinase A (PKA). As a result, PTH cannot effectively stimulate calcium release from bone and phosphorus excretion in the kidneys, leading to hypocalcemia and hyperphosphatemia.
**Why Each Wrong Option is Incorrect**
* **Option A:** Pseudohypoparathyroidism is not characterized by an overproduction of PTH, but rather by resistance to its effects.
* **Option B:** The condition is not caused by a deficiency of vitamin D, but rather by a defect in PTH signaling.
* **Option C:** Pseudohypoparathyroidism is not associated with an increased risk of osteoporosis, but rather with a decrease in bone mineral density due to impaired PTH action.
**Clinical Pearl / High-Yield Fact**
Pseudohypoparathyroidism is often associated with Albright's hereditary osteodystrophy, a condition characterized by short stature, round face, and brachydactyly (short fingers).
**Correct Answer: A. Pseudohypoparathyroidism is characterized by resistance to parathyroid hormone (PTH), leading to hypocalcemia and hyperphosphatemia.**