Inheritence of Familial hypophosphatemic Rickets is
**Core Concept**
Familial hypophosphatemic rickets (FHR) is a rare genetic disorder characterized by impaired renal phosphate reabsorption, leading to hypophosphatemia, growth retardation, and rickets. This condition is caused by mutations in the phosphate-regulating gene, which affects the expression of the sodium-phosphate cotransporter type 2a (NaPi2a) in the proximal renal tubules.
**Why the Correct Answer is Right**
FHR is inherited in an X-linked dominant pattern, meaning that the mutated gene is located on the X chromosome and only one copy of the mutated gene is necessary to cause the condition. Female carriers have a 50% chance of passing the mutated gene to their offspring, while male carriers are usually affected due to the absence of a second X chromosome. The disease-causing mutation affects the function of the NaPi2a protein, leading to impaired phosphate reabsorption in the kidneys.
**Why Each Wrong Option is Incorrect**
* **Option A:** FHR is not inherited in an autosomal dominant pattern, which would require both parents to be affected for the offspring to be at risk.
* **Option B:** FHR is not caused by a deficiency of vitamin D, but rather by impaired renal phosphate reabsorption.
* **Option C:** FHR is not inherited in an autosomal recessive pattern, which would require two copies of the mutated gene (one from each parent) for the offspring to be affected.
**Clinical Pearl / High-Yield Fact**
FHR is a rare genetic disorder that can be distinguished from other causes of hypophosphatemia, such as vitamin D deficiency, by its characteristic X-linked dominant inheritance pattern and impaired renal phosphate reabsorption.
**Correct Answer: C**