## Core Concept
Hypocalcemia refers to a condition where the calcium levels in the blood are lower than normal. This condition can result from various causes, including vitamin D deficiency, kidney disease, and certain medications. Calcium plays a crucial role in many bodily functions, including bone health, muscle contraction, and nerve function.

## Why the Correct Answer is Right
Prolonged QT interval is indeed a feature of hypocalcemia. Low calcium levels affect the heart's electrical activity, leading to changes in the electrocardiogram (ECG), such as a prolonged QT interval. This occurs because calcium ions play a critical role in the repolarization of the cardiac muscle.

## Why Each Wrong Option is Incorrect
- **Option A:** Muscle cramps and spasms are symptoms of hypocalcemia. Low calcium levels increase muscle excitability, leading to cramps, spasms, and tetany.
- **Option B:** A positive Chvostek's sign is indicative of hypocalcemia. This sign is elicited by tapping the facial nerve against the bone below and in front of the ear, which results in a twitch of the facial muscles in individuals with low calcium levels.
- **Option D:** A prolonged QT interval on an ECG is associated with hypocalcemia.

## Clinical Pearl / High-Yield Fact
A key clinical correlation to remember is that while hypocalcemia can cause a prolonged QT interval, hypercalcemia (elevated calcium levels) can lead to a shortened QT interval. This relationship between calcium levels and cardiac electrical activity is critical for understanding the effects of calcium imbalances on the heart.

## Correct Answer Line
**Correct Answer: D. Shortened QT interval**