**Core Concept**
Hypercalcemia, characterized by elevated serum calcium levels, can lead to various electrocardiographic (ECG) changes. The ECG is a valuable tool for diagnosing and monitoring hypercalcemia, as it can reveal subtle changes in cardiac conduction and repolarization.

**Why the Correct Answer is Right**
In hypercalcemia, the shortened QT interval is a hallmark ECG finding. This occurs due to the accelerated depolarization and repolarization of the cardiac myocytes. The shortened QT interval is caused by the increased intracellular calcium levels, which enhance the activity of the calcium-dependent potassium channels (IKCa and IKATP). As a result, the repolarization phase is shortened, leading to a decreased QT interval. This ECG finding is often seen in patients with hyperparathyroidism, malignancy, and other conditions associated with elevated serum calcium levels.

**Why Each Wrong Option is Incorrect**
**Option A:** Increased QT interval is not typically associated with hypercalcemia. Instead, it is often seen in conditions such as hypocalcemia, hypokalemia, and certain antiarrhythmic medications.

**Option C:** Increased PR interval is not a specific ECG finding in hypercalcemia. While it can occur in various cardiac conduction disorders, it is not a diagnostic criterion for hypercalcemia.

**Option D:** Tall T waves can be seen in various conditions, including hyperkalemia and myocardial ischemia. However, they are not a characteristic ECG finding in hypercalcemia.

**Clinical Pearl / High-Yield Fact**
When interpreting ECGs in patients with suspected hypercalcemia, remember that a shortened QT interval is a reliable indicator of elevated serum calcium levels. This ECG finding can help guide further diagnostic testing and management.

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