**Core Concept**
Hyperparathyroidism is a condition characterized by an overproduction of parathyroid hormone (PTH) by one or more parathyroid glands. Elevated PTH levels lead to increased calcium levels in the blood, which can have various effects on the body.

**Why the Correct Answer is Right**
Diarrhea is not a typical feature of hyperparathyroidism. Instead, patients often present with constipation due to increased calcium absorption in the gut. The primary effects of hyperparathyroidism include increased serum calcium levels, decreased serum phosphate levels due to increased renal excretion, and the potential for kidney stones (nephrocalcinosis) due to calcium deposition in the kidneys. The increased calcium levels can also lead to bone resorption and osteoporosis.

**Why Each Wrong Option is Incorrect**
**Option A:** Increased serum calcium is a hallmark of hyperparathyroidism due to the stimulatory effects of PTH on calcium release from bone, absorption from the gut, and reabsorption in the kidneys.
**Option B:** Decreased serum phosphate is a consequence of increased PTH, which promotes phosphate excretion in the urine, leading to hypophosphatemia.
**Option D:** Nephrocalcinosis is a complication of hyperparathyroidism, resulting from the deposition of calcium salts in the renal parenchyma, often leading to chronic kidney disease.

**Clinical Pearl / High-Yield Fact**
In patients with hyperparathyroidism, it is essential to monitor calcium and phosphate levels regularly, as well as kidney function, to prevent complications such as nephrocalcinosis and osteoporosis.

**✓ Correct Answer: C. Diarrhea. Diarrhea is not a typical feature of hyperparathyroidism.**