Which of the following sets of biochemical findings is consistent with Vitamin D deficient rickets?

• A. Low or normal serum calcium; Low or normal serum phosphorus; High alkaline phosphatase; Increased parathyroid hormone; Low 25(OH) Vitamin D; Low to normal 1,25(OH) Vitamin D
• B. High calcium; High phosphorus; Low alkaline phosphatase; Decreased parathyroid hormone; High 25(OH) Vitamin D; High 1,25(OH) Vitamin D
• C. Low calcium; High phosphorus; Normal alkaline phosphatase; Normal parathyroid hormone; High 25(OH) Vitamin D; High 1,25(OH) Vitamin D
• D. Normal calcium; Normal phosphorus; Normal alkaline phosphatase; Normal parathyroid hormone; Normal vitamin D

Answer: (D)

Vitamin D deficiency impairs mineralization of growing bone by reducing calcium and phosphate absorption, which lowers the minerals available for bone formation. This commonly produces low or low-normal calcium and low phosphate, and the body responds with secondary hyperparathyroidism to try to maintain calcium levels. Parathyroid hormone increases bone turnover and promotes phosphate wasting by the kidneys, so serum phosphate stays low even as calcium is fought for. The increased bone activity shows up as elevated alkaline phosphatase. The vitamin D status itself is reduced, with low 25(OH) vitamin D; 1,25(OH)2D can be low, or in early deficiency may be normal or only mildly reduced due to PTH stimulation, but it is not a normal, unaltered profile.

So the pattern consistent with vitamin D deficient rickets is low calcium, low phosphate, high alkaline phosphatase, high parathyroid hormone, and low 25(OH) vitamin D. A completely normal pattern across all these values would not reflect the deficiency.