## **Core Concept**
The question presents a case of a 5-year-old boy with bowing of legs, low serum phosphate, normal serum calcium, normal serum alkaline phosphatase (SAP), and normal serum parathyroid hormone (PTH) levels. This clinical scenario suggests a diagnosis related to abnormal bone mineralization or a disorder of phosphate metabolism.

## **Why the Correct Answer is Right**
The key features in this case are the low serum phosphate (1 mg%), normal serum calcium, normal SAP, and normal PTH levels in a child presenting with bowing of legs. These findings are characteristic of **X-linked Hypophosphatemia (also known as Vitamin D-resistant rickets)**, a genetic disorder that leads to low phosphate levels due to impaired renal phosphate reabsorption. The normal PTH level helps to differentiate it from conditions where PTH is elevated, such as primary hyperparathyroidism. The X-ray of the wrist joint likely shows signs of rickets, such as widening of the growth plates and cupping of the metaphyses, but the specific radiographic features are not described here.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is not provided, but typically, conditions like nutritional rickets would have low vitamin D levels, and often, low calcium levels or elevated PTH.
- **Option B:** Similarly, not provided, but disorders like renal tubular acidosis could present with phosphate wasting but often have associated electrolyte imbalances and metabolic acidosis.
- **Option D:** Without specifics, it's hard to address directly, but conditions like hypoparathyroidism would present with low PTH, hyperphosphatemia, and sometimes hypocalcemia.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that in **X-linked Hypophosphatemia**, the condition is resistant to vitamin D treatment due to the underlying defect in phosphate reabsorption. Thus, management often involves phosphate supplements and sometimes active vitamin D analogs.

## **Correct Answer:** C.