A 10-year-old boy has a low plasma ionized Ca2+ and an increased parathyroid hormone level. He has a high plasma alkaline phosphatase, exhibits muscle weakness, and has skeletal areas that are poorly calcified. You prescribe vitamin D3, but see no improvement. However, treatment with 1,25(OH)2-vitamin D3 causes the patient's ionized Ca2+ level to increase. What would the most likely diagnosis be? A) Type 1 vitamin D-dependent rickets B) Rickets due to vitamin D deficiency C) Inability to form 24(OH)-vitamin D₃ D) Receptors for dehydrocholesterol are absent E) Inability to form vitamin D₃ in the skin

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The most likely diagnosis is Type 1 vitamin D-dependent rickets, which is characterized by the body's inability to convert vitamin D to its active form, 1,25(OH)2-vitamin D3. The improvement with 1,25(OH)2-vitamin D3 treatment confirms this condition.

A 10-Year-Old Boy Has a Low Plasma Ionized Ca2+ and an Increased