**Question:** Hyperparathyroidism causes:

**Core Concept:** Hyperparathyroidism is a disorder characterized by the overproduction of parathyroid hormone (PTH) by the parathyroid glands, leading to increased calcium levels in the blood. The primary role of PTH is to regulate calcium and phosphorus homeostasis by affecting bones, kidneys, and the gastrointestinal tract.

**Why the Correct Answer is Right:** In hyperparathyroidism, the overproduction of PTH leads to an increased calcium level in the blood, known as hypercalcemia. This is due to the following effects of PTH:

1. **Increased bone resorption:** PTH stimulates osteoclast activity, causing the release of calcium from bone matrix into the bloodstream.
2. **Increased renal calcium reabsorption:** PTH enhances calcium reabsorption in the nephrons, reducing the excretion of calcium in the urine.
3. **Decreased renal phosphorus reabsorption:** PTH decreases phosphorus reabsorption in the nephrons, leading to increased phosphorus excretion, counteracting the increased calcium absorption.

**Why Each Wrong Option is Incorrect:**

A. **Decreased renal calcium reabsorption:** This is incorrect because PTH increases renal calcium reabsorption, as mentioned earlier.
B. **Increased renal phosphate reabsorption:** This is incorrect because PTH decreases renal phosphate reabsorption, as explained above.
C. **Increased bone resorption:** Although hyperparathyroidism does lead to increased bone resorption, this is not the primary cause of hypercalcemia in hyperparathyroidism. The main cause is the increased PTH production.
D. **Increased renal calcium excretion:** This is incorrect because PTH increases renal calcium reabsorption, leading to decreased excretion of calcium.

**Clinical Pearl:**

Hyperparathyroidism should be considered in patients presenting with unexplained hypercalcemia and/or renal stones. A thorough clinical assessment, including a review of systems and physical examination, is crucial in making a correct diagnosis.

**Correct Answer:** C. Increased bone resorption