Osteomalacia and Rickets

A 3-year-old child from rural Bihar presents with bowing of legs, delayed fontanelle closure, and frontal bossing. Serum calcium is 7.2 mg/dL, phosphate 3.5 mg/dL, and alkaline phosphatase is markedly elevated. What is the most common cause of rickets in this clinical scenario?

A 35-year-old woman with chronic kidney disease stage 4 (eGFR 22 mL/min/1.73 m²) presents with bone pain, muscle weakness, and a serum calcium of 7.8 mg/dL (corrected). Laboratory studies show phosphate 5.2 mg/dL, alkaline phosphatase 420 IU/L, PTH 580 pg/mL (normal 15–65), and 25-hydroxyvitamin D 28 ng/mL. 1,25-dihydroxyvitamin D level is 18 pg/mL (normal 20–60). What is the most appropriate immediate next step in management?

In nutritional rickets, which of the following biochemical findings occurs EARLIEST in the disease process?

A 38-year-old woman from rural Bihar presents with progressive bone pain, muscle weakness, and difficulty climbing stairs for the past 8 months. She reports minimal sun exposure due to cultural practice of purdah. On examination, she has proximal muscle weakness (grade 4/5), bone tenderness over the tibia and femur, and a waddling gait. Serum calcium is 7.2 mg/dL, phosphate 2.8 mg/dL, alkaline phosphatase 120 IU/L (elevated), and 25-hydroxyvitamin D is 12 ng/mL. X-ray shows loss of sharp metaphyseal margins and Looser's zones in the femur. What is the most likely diagnosis?

Which of the following is the primary defect in vitamin D-dependent rickets type 1 (VDDR-1)?