In a pediatric patient with osteopenia and fractures, low calcium, low phosphorus, high alkaline phosphatase, low 25-OH vitamin D, low 1,25 (OH)2 vitamin D, and high PTH, the most likely diagnosis is which?

In a pediatric patient with osteopenia and fractures, low calcium, low phosphorus, high alkaline phosphatase, low 25-OH vitamin D, low 1,25 (OH)2 vitamin D, and high PTH, the most likely diagnosis is which?

• A. Renal tubular acidosis
• B. Osteogenesis imperfecta
• C. Vitamin D deficiency rickets
• D. Vitamin D dependent rickets type 2

Answer: (C)

This pattern centers on how insufficient vitamin D disrupts mineral balance and bone mineralization in kids. When vitamin D stores are low, gut absorption of calcium and phosphate drops, leading to low calcium and low phosphate in the blood. In response, parathyroid hormone rises (secondary hyperparathyroidism) to keep calcium up, which increases bone resorption and renal calcium reabsorption but also promotes phosphate wasting, so phosphate remains low. The ongoing need for bone mineralization drives osteoblastic activity, shown by a high alkaline phosphatase. Because the substrate for forming active vitamin D is scarce, the 25-OH vitamin D level is low, and the downstream 1,25(OH)2 vitamin D level is also low or inappropriately low. This constellation—low calcium, low phosphate, high alkaline phosphatase, low 25-OH vitamin D, low 1,25(OH)2 vitamin D, and elevated PTH—fits vitamin D deficiency–related rickets in a pediatric patient with osteopenia and fractures. The other conditions don’t match as well. Osteogenesis imperfecta typically shows normal mineral metabolism labs with fractures from collagen defects. Renal tubular acidosis would involve acidosis and electrolyte disturbances without this specific vitamin D depletion pattern. Vitamin D dependent rickets type 2 involves resistance to 1,25(OH)2 vitamin D and usually features elevated 1,25(OH)2 vitamin D levels or altered receptor signaling, not low levels across both vitamin D metabolites.

This pattern centers on how insufficient vitamin D disrupts mineral balance and bone mineralization in kids. When vitamin D stores are low, gut absorption of calcium and phosphate drops, leading to low calcium and low phosphate in the blood. In response, parathyroid hormone rises (secondary hyperparathyroidism) to keep calcium up, which increases bone resorption and renal calcium reabsorption but also promotes phosphate wasting, so phosphate remains low. The ongoing need for bone mineralization drives osteoblastic activity, shown by a high alkaline phosphatase. Because the substrate for forming active vitamin D is scarce, the 25-OH vitamin D level is low, and the downstream 1,25(OH)2 vitamin D level is also low or inappropriately low. This constellation—low calcium, low phosphate, high alkaline phosphatase, low 25-OH vitamin D, low 1,25(OH)2 vitamin D, and elevated PTH—fits vitamin D deficiency–related rickets in a pediatric patient with osteopenia and fractures.

The other conditions don’t match as well. Osteogenesis imperfecta typically shows normal mineral metabolism labs with fractures from collagen defects. Renal tubular acidosis would involve acidosis and electrolyte disturbances without this specific vitamin D depletion pattern. Vitamin D dependent rickets type 2 involves resistance to 1,25(OH)2 vitamin D and usually features elevated 1,25(OH)2 vitamin D levels or altered receptor signaling, not low levels across both vitamin D metabolites.