**Core Concept:**
Parathyroid Hormone (PTH) is a hormone secreted by the four parathyroid glands, primarily regulating calcium and phosphorus levels in the blood. It acts on bone, kidney, and intestine to increase calcium levels in the blood and decrease phosphorus levels.

**Why the Correct Answer is Right:**
PTH primarily acts on bone and kidney to raise calcium levels. It stimulates osteoclasts to release calcium from bones, increasing serum calcium concentration. It also enhances calcium reabsorption in the kidneys, reducing excretion and subsequently increasing serum calcium levels. Additionally, PTH inhibits phosphate reabsorption in the kidneys, which also contributes to the decrease in phosphorus levels.

**Why Each Wrong Option is Incorrect:**
A. Phosphorus reabsorption in the kidneys is directly inhibited by PTH, not stimulated. Therefore, option A is incorrect.
B. PTH increases calcium levels in the blood, not decrease them. Option B is incorrect.
C. PTH stimulates bone resorption and renal calcium reabsorption, not inhibit them. Option C is incorrect.
D. PTH directly affects the intestine to promote calcium absorption, not decrease it. Option D is incorrect.

**Clinical Pearl / High-Yield Fact:**
A balanced calcium and phosphorus levels are essential for proper bone mineralization and overall physiological functioning. Abnormal calcium and phosphorus levels can lead to various diseases like osteoporosis, hypercalcemia, and hypophosphatemia. Adequate understanding of PTH actions is crucial for maintaining these levels and diagnosing and treating associated disorders.

**Correct Answer:**
Correct Answer: D. PTH stimulates intestinal calcium absorption.

Explanation: PTH directly affects the intestine, promoting the absorption of calcium ions from the GI tract into the bloodstream, thus increasing serum calcium levels. This is an essential mechanism for maintaining calcium homeostasis and ensuring proper bone mineralization.