A 72-year-old woman is evaluated for a 3-year history of progressively worsening low back pain involving the lumbar spine, sacroiliac joints, and hips. She reports progressive difficulty with rising from a squat and climbing stairs. She also has had several spontaneous fractures over the past year. At the time of presentation, she cannot walk without support. Medical history is also notable for type 2 diabetes mellitus, hypertension, and hyperlipidemia. Medications are glipizide, quinapril, rosiglitazone, atorvastatin, and hydrocodone/acetaminophen.

On physical examination, blood pressure is 147/84 mm Hg, and pulse rate is 82/min. There is a small swelling over the proximal phalanx of the left index finger. There is tenderness upon palpation of the ribs. Examination of the joints and spine is normal. Decreased strength in the proximal muscles of the lower limbs is noted.

Laboratory studies:

Albumin

3.9 g/dL (39 g/L)

Alkaline phosphatase

436 U/L

Calcium

9.0 mg/dL (2.3 mmol/L)

Creatinine

0.9 mg/dL (79.6 µmol/L)

Phosphorus

1.7 mg/dL (0.55 mmol/L)

Intact parathyroid hormone

22 pg/mL (22 ng/L)

1,25-Dihydroxy vitamin D

5.0 pg/mL (12 pmol/L)

25-Hydroxy vitamin D

40 ng/mL (99.8 nmol/L)

24-Hour urine phosphate

1.4 g/24 h (45 mmol/24 h) (normal range, 0.4-1.3 g/24 h [12.9-42 mmol/24 h])

Which of the following is the most likely cause of this patient's hypophosphatemia?