**Core Concept**
Rickets is a disease characterized by softening of the bones in children, caused by a deficiency of vitamin D, calcium, or phosphate. The diagnosis of rickets is based on clinical presentation, laboratory investigations, and radiological findings.

**Why the Correct Answer is Right**
The child in the question has symptoms of rickets, and the laboratory investigations show a low serum phosphate level (2.3 mg/dl) and an elevated alkaline phosphatase level (2420 IU/L). These findings are consistent with a diagnosis of nutritional rickets, which is caused by a deficiency of vitamin D and/or calcium in the diet. Vitamin D is essential for the absorption of calcium and phosphate from the intestine, and a deficiency can lead to a decrease in serum calcium and phosphate levels. The elevated alkaline phosphatase level is a marker of bone turnover, which is increased in rickets.

**Why Each Wrong Option is Incorrect**
**Option A:** Vitamin D-dependent rickets type II is caused by a defect in the 1-alpha-hydroxylase enzyme, which converts 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D. This condition typically presents with severe hypercalcemia and is less common than nutritional rickets.

**Option B:** Hypophosphatemic rickets is caused by a defect in the phosphate reabsorption in the kidneys or a genetic mutation affecting phosphate metabolism. This condition is characterized by low serum phosphate levels and is not typically associated with elevated alkaline phosphatase levels.

**Option D:** Secondary hyperparathyroidism is caused by a deficiency of vitamin D and/or phosphate, leading to an increase in parathyroid hormone (PTH) production. While this condition can cause an increase in alkaline phosphatase levels, it is not the most probable diagnosis in this case, as the serum calcium level is not low.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that nutritional rickets is a common cause of rickets in children, and it can be prevented by ensuring adequate vitamin D and calcium intake through diet or supplements. A diet rich in vitamin D and calcium is essential for healthy bone growth and development.

**✓ Correct Answer: C. Nutritional Rickets**