A 38-year-old African-American man comes to your office complaining of diffuse bone pain for the last 6 months. He describes it as a severe deep ache involving the neck, shoulders, back, hips and thighs. He has been receiving treatment for chronic hepatitis B virus infection. Otherwise, his medical history is unremarkable. Physical examination findings are as follows: He has a dark skin complexion; pale conjunctiva, and dry mucous membranes. The lungs are clear to auscultation. The cardiac examination is notable for an S4 gallop. There is wasting of extremities; pain with palpation of all extremities and ⅗ muscle strength in all extremities. You schedule further tests to try and determine the etiology of this patient’s condition. Laboratory reports show hypocalcemia, hypophosphatemia, and elevated alkaline phosphatase. The x-ray films reveal diffuse radiolucency with thinning of the cortical bone in both femurs, and pseudofractures of the humerus in both right and left upper extremities. Which of the following agents is most likely responsible for his current disorder?
