**Core Concept**
Hypophosphataemic rickets is a rare genetic disorder characterized by phosphate wasting, leading to softening of bones in children, and osteomalacia in adults. This condition is caused by mutations in genes regulating phosphate reabsorption in the kidneys.

**Why the Correct Answer is Right**
Hypophosphataemic rickets is primarily caused by mutations in the PHEX gene, which encodes a protein involved in phosphate reabsorption in the kidneys. The condition leads to impaired phosphate reabsorption, resulting in excessive phosphate excretion in the urine. This, in turn, causes a decrease in phosphate levels in the blood, leading to the characteristic symptoms of the disease, including softening of bones and growth retardation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as hypophosphataemic rickets is not caused by a deficiency in vitamin D, but rather by a genetic mutation affecting phosphate reabsorption in the kidneys.
**Option B:** This option is incorrect as hyperphosphataemia is not a characteristic of hypophosphataemic rickets; instead, phosphate levels are decreased in the blood.
**Option C:** This option is incorrect as hypophosphataemic rickets is not caused by an excessive excretion of phosphate in the urine; rather, it is caused by impaired phosphate reabsorption in the kidneys.

**Clinical Pearl / High-Yield Fact**
A key feature of hypophosphataemic rickets is the presence of normal or elevated levels of 1,25-dihydroxyvitamin D, despite the presence of phosphate wasting. This is because the kidneys are able to produce active vitamin D in response to low phosphate levels, even in the presence of impaired phosphate reabsorption.

**Correct Answer:** C.