**Core Concept**
Vitamin D plays a crucial role in bone health by regulating calcium and phosphate metabolism. A deficiency in vitamin D can lead to rickets in children, characterized by softening of the bones and skeletal deformities.

**Why the Correct Answer is Right**
The patient's symptoms of deformed legs and elevated alkaline phosphatase levels are indicative of rickets, a condition caused by vitamin D deficiency. Vitamin D is essential for the absorption of calcium and phosphate from the gut, and its deficiency leads to impaired bone mineralization. Alkaline phosphatase is an enzyme involved in bone mineralization, and its elevation is a marker of bone turnover and rickets. In strict vegetarians, vitamin D deficiency is common due to limited dietary sources and reduced sun exposure.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it does not directly relate to the patient's symptoms and lab findings.
**Option B:** This option is incorrect as vitamin B12 deficiency primarily affects the hematological system, not bone health.
**Option C:** This option is incorrect as vitamin C deficiency (scurvy) primarily affects collagen synthesis and does not lead to elevated alkaline phosphatase levels.

**Clinical Pearl / High-Yield Fact**
A strict vegetarian patient with rickets may require vitamin D supplementation, and it is essential to assess their calcium and phosphate levels, as well as their parathyroid hormone (PTH) levels, to manage the condition effectively.

**Correct Answer:** D.