**Core Concept**
Vitamin D plays a crucial role in maintaining calcium and phosphate homeostasis in the body. It regulates the expression of genes involved in calcium absorption and metabolism, and its deficiency can lead to changes in bone mineralization and turnover.

**Why the Correct Answer is Right**
In Vitamin D deficiency, the body attempts to compensate for the lack of calcium absorption by increasing the activity of osteoclasts and osteoblasts, leading to an increase in bone turnover. This results in elevated levels of alkaline phosphatase (ALP) in the blood, as ALP is an enzyme involved in the mineralization of bone. Specifically, the liver isoenzyme of ALP (L-ALP) is increased in response to Vitamin D deficiency.

**Why Each Wrong Option is Incorrect**
**Option B:** Decreased phosphate in urine is incorrect because Vitamin D deficiency leads to decreased phosphate absorption from the gut, resulting in hyperphosphaturia (increased phosphate excretion in the urine).

**Option C:** Hyperphosphatemia is incorrect because, although Vitamin D deficiency leads to decreased phosphate absorption from the gut, the body attempts to compensate by increasing phosphate reabsorption in the kidneys, which can lead to hyperphosphatemia.

**Option D:** Decreased alkaline phosphatase is incorrect because, as explained earlier, Vitamin D deficiency leads to increased bone turnover and elevated ALP levels.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that elevated alkaline phosphatase levels can be seen in conditions other than Vitamin D deficiency, such as bone metastases, Paget's disease, and certain liver disorders. A thorough clinical evaluation and review of the patient's history and laboratory results are necessary to determine the underlying cause of elevated ALP.

**✓ Correct Answer: A. Increased alkaline phosphatase**