A 79-year-old woman presents to the emergency department (ED) with a chief complaint of a 3-day history of left hip pain. She only mildly twisted her left leg 3 days ago; she has since had increasing pain and difficulty walking. The pain is sharp and radiates down to her left knee. It is mild when she is at rest, but it becomes severe when she attempts to walk. She has not had any weakness or numbness. She denies having any fevers. She has not had any direct trauma to the hip or falls, and there has not been any notable swelling of the leg or skin changes/rash. The review of systems also reveals that she has unintentionally lost 8.8 lb (4.0 kg) over the last 6 months. She denies having any nausea, vomiting, night sweats, cough, or shortness of breath. She has a history of multiple rib fractures that resulted from vigorous coughing 2 years ago; at that time, she was diagnosed only with osteoporosis as the cause of these fractures. She denies having any previous fractures otherwise. Her past medical history also includes chronic obstructive pulmonary disease (COPD), although she has never been a smoker. She also has known osteoarthritis of the hips, scoliosis of the thoracic spine, and a presumptive diagnosis of Paget disease on the basis of a single elevated serum alkaline phosphatase. Her medications include budesonide/eformoterol and terbutaline inhalers, oral calcium and vitamin D supplements, and weekly alendronate. A bisphosphonate was started after dual-energy x-ray absorptiometry (DXA) showed a T score of -2, which is consistent with osteopenia. Both her mother and sister had broken their hips later in their lives. She lives independently and is still driving. She denies experiencing physical abuse, and there are several family members accompanying her in the ED who show concern for her.
On physical examination, the patient appears well and is lying comfortably in bed. Her heart rate is 84 bpm and regular, with a blood pressure of 140/80 mm Hg. Her temperature is normal at 98.9°F (37.2°C), and her respiratory rate is 14 breaths/min. Her cardiovascular, respiratory, and abdominal examinations are all normal. She has moderate pain with any movement of the left leg and has groin tenderness on palpation. The left leg is shortened and externally rotated. Her peripheral pulses are palpable, and she has normal distal strength and sensation in the left lower extremity.
Her blood tests reveal a normal complete blood cell (CBC) count; however, her electrolytes show a significantly depleted phosphate concentration of 0.9 mg/dL (0.29 mmol/L; normal range, 3-4.5 mg/dL). Additional laboratory results include a creatinine of 0.68 mg/dL (60 μmol/L; normal range, <1.5 mg/dL), adjusted calcium of 8.88 mg/dL (2.22 mmol/L; normal range, 9.0–10.5 mg/dL), albumin of 3.6 g/dL (36 g/L; normal range, 3.5-5.5 g/dL), alkaline phosphatase of 350 U/L (normal range, 30-120 U/L), vitamin D (25-hydroxy) of 22.04 ng/mL (55 nmol/L; normal range, 10-68 ng/mL), and a parathyroid hormone of 107.3 pg/mL (107.3 ng/L; normal range, 10-60 pg/mL).
A chest radiograph reveals multiple healed rib fractures. Radiographs of the pelvis and left hip are obtained as well, which reveal a left femoral neck fracture.
What is the underlying disorder that resulted in the patient's pathologic hip fracture?
Hint: The serum phosphate is low, but the serum calcium is preserved.
Vitamin D deficiency
Oncogenic osteomalacia
Fanconi syndrome
Secondary hyperparathyroidism
