A 55-year-old man with a 20-year history of type 2 diabetes mellitus presents for evaluation of fatigue and muscle cramps. He has stage 4 chronic kidney disease with an estimated glomerular filtration rate of 25 mL/min/1.73 m2. Laboratory studies reveal serum calcium 7.8 mg/dL (normal 8.5–10.5 mg/dL), phosphate 5.2 mg/dL (normal 2.5–4.5 mg/dL), PTH 380 pg/mL (normal 15–65 pg/mL), 25-hydroxyvitamin D 32 ng/mL (normal 20–50 ng/mL), and 1,25-dihydroxyvitamin D 10 pg/mL (normal 20–60 pg/mL). Which of the following enzymatic steps in vitamin D metabolism is most likely impaired in this patient?