**Core Concept**
Hypocalcemia in children can be caused by various factors, including genetic disorders, nutritional deficiencies, and abnormalities in calcium metabolism. The body tightly regulates calcium levels through a complex interplay of hormones, including parathyroid hormone (PTH) and vitamin D.

**Why the Correct Answer is Right**
One of the primary causes of hypocalcemia in children is vitamin D deficiency. Vitamin D plays a crucial role in calcium absorption from the gut, and its deficiency can lead to rickets in children. Rickets is characterized by softening of the bones, which can cause hypocalcemia. Additionally, vitamin D deficiency can also lead to secondary hyperparathyroidism, which further exacerbates hypocalcemia. Vitamin D is obtained from dietary sources and sunlight exposure, and its deficiency is common in children who have limited sun exposure or a poor diet.

**Why Each Wrong Option is Incorrect**
**Option A:** While phosphate levels are often elevated in hypocalcemia, phosphate itself is not a direct cause of hypocalcemia.
**Option B:** Pancreatitis can lead to hypocalcemia, but it is not a common cause in children. Pancreatitis is more often associated with hypercalcemia due to the release of calcium from the pancreas.
**Option C:** While certain medications, such as furosemide, can cause hypocalcemia, they are not the most common causes in children.

**Clinical Pearl / High-Yield Fact**
One key point to remember is that vitamin D deficiency can lead to secondary hyperparathyroidism, which can further exacerbate hypocalcemia. This is because PTH acts to increase calcium levels by stimulating its release from bones and increasing its absorption from the gut.

**Correct Answer:** D. Vitamin D deficiency.