**Core Concept**
Osteomalacia is a metabolic bone disease characterized by the softening of bones due to impaired bone mineralization. It occurs when the body is unable to maintain adequate levels of calcium and phosphate, leading to the accumulation of unmineralized osteoid tissue in the bone matrix.

**Why the Correct Answer is Right**
The correct answer is related to the pathophysiology of osteomalacia, which involves the impaired activity of the enzyme 1-alpha-hydroxylase. This enzyme is responsible for converting 25-hydroxyvitamin D to its active form, 1,25-dihydroxyvitamin D, which is essential for calcium absorption and phosphate reabsorption in the kidneys. In osteomalacia, the impaired activity of 1-alpha-hydroxylase leads to decreased levels of 1,25-dihydroxyvitamin D, resulting in impaired bone mineralization.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because hyperphosphatemia is often seen in osteomalacia due to impaired renal phosphate excretion.
**Option B:** This option is incorrect because hypocalcemia is a common feature of osteomalacia due to impaired calcium absorption and mobilization from bone.
**Option C:** This option is incorrect because increased alkaline phosphatase levels are often seen in osteomalacia due to the increased activity of osteoblasts in attempting to mineralize the bone matrix.

**Clinical Pearl / High-Yield Fact**
A key feature of osteomalacia is the presence of a characteristic "bone within a bone" appearance on radiographs, which is due to the accumulation of unmineralized osteoid tissue in the bone matrix.

**Correct Answer:** D