**Core Concept:** Understanding the distribution of plasma calcium concentrations is crucial for understanding calcium homeostasis and its regulation. Calcium is an essential mineral for various cellular functions, and its concentration in the body is tightly regulated.

**Why the Correct Answer is Right:** Calcium is primarily regulated by the parathyroid hormone (PTH), calcitonin, and vitamin D. These hormones play a role in maintaining calcium homeostasis by modulating the reabsorption of calcium in the kidneys and the absorption of calcium from the gastrointestinal tract.

**Why Each Wrong Option is Incorrect:**

A. **Option A:** This answer is incorrect because PTH is the hormone responsible for increasing plasma calcium concentration. It stimulates osteoclasts to release calcium from bone and increases renal calcium reabsorption.

B. **Option B:** Although calcitonin decreases plasma calcium concentration, the correct answer should be the opposite, as calcitonin lowers calcium levels by inhibiting osteoclast activity and reducing renal calcium reabsorption.

C. **Option C:** Vitamin D is responsible for increasing calcium absorption in the gastrointestinal tract but does not directly affect plasma calcium concentration.

D. **Option D:** Vitamin D is actually involved in regulating calcium absorption, not decreasing plasma calcium concentration. However, it is less involved in the direct regulation of plasma calcium concentration compared to PTH and calcitonin.

**Clinical Pearl / High-Yield Fact:** A high plasma calcium level is associated with conditions like primary hyperparathyroidism, while a low plasma calcium level is seen in hypoparathyroidism or hypovitaminosis D. Maintaining the balance of calcium levels is crucial for ensuring proper muscle, nerve, and bone function.

**Correct Answer:** D. Vitamin D is responsible for increasing calcium absorption and plays a significant role in regulating calcium homeostasis. However, it is not directly involved in the regulation of plasma calcium concentration as much as parathyroid hormone (PTH) and calcitonin.