**Core Concept**
Calcium and phosphorus metabolism is a tightly regulated process that involves the interplay of various hormones, including parathyroid hormone (PTH), vitamin D, and calcitonin. This complex system maintains the balance of calcium and phosphorus levels in the body, which is crucial for bone mineralization, muscle function, and nerve transmission.

**Why the Correct Answer is Right**
The correct answer involves the regulation of calcium and phosphorus levels by PTH and vitamin D. PTH stimulates the release of calcium from bones, increases renal calcium reabsorption, and enhances vitamin D-mediated intestinal calcium absorption. Vitamin D, in turn, promotes the absorption of calcium from the intestine by increasing the expression of calbindin, a calcium-binding protein. This intricate mechanism ensures that calcium levels remain within a narrow range, essential for maintaining bone health and preventing conditions such as hypocalcemia and hypercalcemia.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the relationship between PTH and vitamin D in regulating calcium and phosphorus metabolism.
**Option B:** This option is incorrect because it implies that vitamin D has a direct effect on phosphorus levels, whereas its primary role is to regulate calcium absorption.
**Option C:** This option is incorrect because it suggests that calcitonin has a primary role in regulating calcium and phosphorus metabolism, whereas it actually plays a minor role in reducing blood calcium levels.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that vitamin D deficiency can lead to impaired calcium absorption, resulting in secondary hyperparathyroidism and increased bone resorption. This can manifest as osteomalacia in adults and rickets in children.

**Correct Answer:** D.