## Core Concept
The parathyroid hormone (PTH) is a **polypeptide hormone** secreted by the parathyroid glands that plays a crucial role in regulating calcium and phosphate levels in the blood. It acts on bones, kidneys, and the intestine (indirectly) to maintain calcium homeostasis. The secretion of PTH is primarily regulated by the serum calcium concentration.

## Why the Correct Answer is Right
PTH acts to increase serum calcium levels through several mechanisms:
- It stimulates **osteoclasts** in bones to release calcium into the bloodstream.
- In the kidneys, it promotes **calcium reabsorption** and increases the excretion of phosphate by inhibiting phosphate reabsorption.
- In the intestine, PTH indirectly increases calcium absorption by stimulating the kidneys to produce **1,25-dihydroxyvitamin D (calcitriol)**, which enhances intestinal absorption of calcium.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because PTH does not decrease serum calcium levels; instead, it works to increase them.
- **Option B:** This option is incorrect as PTH does not primarily act by decreasing phosphate levels directly but does so as a secondary effect by promoting phosphate excretion in the urine.
- **Option C:** Without the specific details of this option, we can infer based on the correct answer that it does not accurately describe the primary action or effect of PTH.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that **hyperparathyroidism**, which results in elevated PTH levels, can lead to **hypercalcemia**, bone resorption, and kidney stones. Conversely, **hypoparathyroidism** leads to **hypocalcemia** and **hyperphosphatemia**.

## Correct Answer Line
**Correct Answer: D.**