**Question:** A 60-year-old male presents with a bony abnormality at the upper tibia associated with sensorineural hearing loss. His laboratory examination reveals elevated serum alkaline phosphatase levels (440 mU/1) and normal serum calcium (Ca2+) and phosphorus (P042-). A skeletal survey reveals ivory vertebrae and cotton wool spots in the skull x-ray. What is the diagnosis?

**Core Concept:** This question is testing the understanding of hypophosphatemic rickets, a rare cause of osteomalacia and sensorineural hearing loss. Hypophosphatemic rickets is a group of genetic or acquired disorders characterized by low serum phosphate levels leading to hypophosphatemia.

**Core Concept Explanation:** Alkaline phosphatase is an enzyme that plays a role in calcium and phosphate homeostasis. Elevated alkaline phosphatase levels are present in cases of bone resorption, like hypophosphatemic rickets. Normal calcium and phosphorus levels exclude hypocalcemia and hyperparathyroidism as potential causes. Cotton wool spots on skull X-ray indicate hypophosphatemia, while ivory vertebrae suggest a longstanding process.

**Why the Correct Answer is Right:**

- The correct answer (D) is "Hypophosphatemic rickets," as the patient's clinical presentation, laboratory findings, and imaging results are consistent with this diagnosis.

**Why Other Options are Incorrect:**

A. Vitamin D deficiency rickets (Option A) is characterized by hypercalcemia and elevated alkaline phosphatase levels, not hypophosphatemia and elevated alkaline phosphatase levels.

B. Rickets (Option B) is a broad term for childhood bone deformities due to vitamin D deficiency, which is not applicable in this case.

C. Osteomalacia (Option C) is a condition of softening of bones due to defective bone mineralization, but does not explain the sensorineural hearing loss and the specific findings of hypophosphatemic rickets.

**Clinical Pearl:** Hypophosphatemic rickets should be considered in patients presenting with sensorineural hearing loss, low serum phosphate levels, and specific radiographic findings. Hypophosphatemic rickets can be primary (e.g., hereditary or medication-induced) or secondary (e.g., renal phosphate wasting disorders). Early diagnosis and treatment can prevent further bone deformities and improve the patient's quality of life.