**Core Concept**
Hyperparathyroidism is characterized by excessive parathyroid hormone (PTH) secretion, leading to increased serum calcium and bone resorption. Key features include elevated serum calcium, increased urinary calcium, and bone changes such as subperiosteal resorption. Calcitonin, produced by the thyroid C-cells, lowers serum calcium and is typically suppressed in hyperparathyroidism.

**Why the Correct Answer is Right**
In hyperparathyroidism, PTH stimulates osteoclast activity, increasing serum calcium and urinary calcium excretion. It also causes subperiosteal resorption, especially in the phalanges. Serum calcitonin is normally low or suppressed in hyperparathyroidism due to the opposing effect of PTH on calcium regulation. Thus, a *decrease* in serum calcitonin is not a marker of hyperparathyroidism — rather, it reflects the physiological response to high calcium, not a direct marker.

**Why Each Wrong Option is Incorrect**
Option A: An increase in serum calcium is a hallmark of hyperparathyroidism due to PTH-mediated bone resorption and renal calcium reabsorption.
Option B: Increased 24-hour urinary calcium excretion occurs because PTH enhances calcium reabsorption in the kidneys and increases calcium release from bone.
Option D: Subperiosteal resorption of phalanges is a classic radiological sign of chronic hyperparathyroidism, especially in older adults.

**Clinical Pearl / High-Yield Fact**
Remember: In hyperparathyroidism, serum calcium rises, urinary calcium rises, and calcitonin *decreases* — so a *decrease* in calcitonin is a *consequence*, not a marker. Always look for *elevated* calcium and *increased* urinary calcium as primary markers.

✓ Correct Answer: C. Decrease in serum calcitonin