**Core Concept**
Hypercalcemia, a condition characterized by elevated calcium levels in the blood, can have significant effects on the cardiac conduction system. The electrocardiogram (ECG) is a valuable tool for diagnosing and monitoring electrolyte imbalances, including hypercalcemia.

**Why the Correct Answer is Right**
In hypercalcemia, the increased calcium levels affect the cardiac conduction system, leading to a shortening of the QT interval. This is because calcium ions (Ca2+) play a crucial role in the depolarization and repolarization of cardiac cells. Elevated calcium levels enhance the depolarization process, resulting in a shorter QT interval. The QT interval represents the time from the beginning of the Q wave to the end of the T wave in the ECG, and a shortened QT interval is a hallmark of hypercalcemia.

**Why Each Wrong Option is Incorrect**
**Option A:** An increased QT interval is actually a feature of hypocalcemia, not hypercalcemia. Hypocalcemia prolongs the QT interval due to reduced calcium availability for cardiac cell depolarization.

**Option C:** While hypercalcemia can affect various cardiac conduction intervals, the PR interval is not typically shortened. The PR interval represents the time from the beginning of the P wave to the start of the QRS complex, and its length is more closely related to atrioventricular (AV) node conduction.

**Option D:** Tall T waves can be associated with several conditions, including hyperkalemia, but not typically with hypercalcemia. In hypercalcemia, the T wave can be normal or even shortened, rather than tallened.

**Clinical Pearl / High-Yield Fact**
Remember, in hypercalcemia, a shortened QT interval is a key ECG finding. This can help differentiate hypercalcemia from other electrolyte imbalances, such as hypocalcemia, which prolongs the QT interval.

**✓ Correct Answer: B. Decreased QT interval**