Causes of Hypocalcemia in a child are-: (A) DiGeorge syndrome, (B) Hypoparathyroidism, (C) Magnesium deficiency, (D) All of above
Answer is D. Causes of hypocalcemia 

Hypoparathyroidism
DiGeorge syndrome
PTH receptor defect (pseudohypoparathyroidism)
Magnesium deficiency
Exogenous organic phosphate excess
Vit D difecideciency