A 48-year-old man presents to the clinic with a 7-month history of progressive, deep bone pain in his ribs, hips, and lower extremities, along with significant proximal muscle weakness. He reports difficulty climbing stairs and rising from a seated position. He has no significant past medical history, takes no medications, and consumes a well-balanced diet. He works as a construction manager and spends a significant amount of time outdoors.
Vital signs are within normal limits. Physical examination reveals a waddling gait and diffuse bony tenderness upon palpation of the rib cage and anterior pelvis. Neurologic examination shows 4/5 motor strength in the proximal muscles of the bilateral lower extremities with normal deep tendon reflexes.
Laboratory results are as follows:
| Serum Chemistry | Value | Reference Range |
|---|---|---|
| Sodium | 140 mEq/L | 136-145 mEq/L |
| Potassium | 4.1 mEq/L | 3.5-5.0 mEq/L |
| Chloride | 104 mEq/L | 98-106 mEq/L |
| Bicarbonate | 24 mEq/L | 22-28 mEq/L |
| BUN | 14 mg/dL | 8-20 mg/dL |
| Creatinine | 0.9 mg/dL | 0.7-1.3 mg/dL |
| Calcium | 8.7 mg/dL | 8.4-10.2 mg/dL |
| Phosphorus | 1.4 mg/dL | 2.5-4.5 mg/dL |
| Alkaline phosphatase | 380 U/L | 40-120 U/L |
| Additional Studies | Value | Reference Range |
|---|---|---|
| Parathyroid hormone (PTH) | 45 pg/mL | 10-65 pg/mL |
| 25-hydroxyvitamin D | 36 ng/mL | 20-50 ng/mL |
| 1,25-dihydroxyvitamin D | 11 pg/mL | 18-72 pg/mL |
| Urine FE of phosphorus | 26% | < 5% (in hypophosphatemia) |
Urinalysis is negative for protein, glucose, and blood, with a pH of 6.0.
Which of the following best describes the primary pathophysiologic mechanism underlying this patient's clinical and laboratory findings?
- AAutonomous hypersecretion of parathyroid hormone
- Ectopic secretion of fibroblast growth factor 23Answer
- CGeneralized dysfunction of the renal proximal tubule
- DInactivating mutation of the PHEX gene
- EInadequate dietary intake and limited cutaneous synthesis of cholecalciferol