**Core Concept**
Hypercalcemia is a condition characterized by elevated calcium levels in the blood, which can lead to various ECG changes due to its effects on cardiac ion channels and membrane potentials. The ECG changes in hypercalcemia are primarily due to the increased calcium influx into cardiac cells, leading to prolongation of the QT interval.

**Why the Correct Answer is Right**
The prolongation of the QT interval in hypercalcemia is due to the increased calcium influx into cardiac cells, which enhances the activity of the L-type calcium channels. This, in turn, prolongs the plateau phase of the action potential, leading to a delayed repolarization and an increased QT interval. The corrected QT interval (QTc) is more accurate in assessing the effect of hypercalcemia on the ECG.

**Why Each Wrong Option is Incorrect**
**Option A:** Prolonged PR interval is not a specific indicator of hypercalcemia, as it can be seen in various conditions, including first-degree atrioventricular (AV) block, hypokalemia, and digitalis toxicity.

**Option B:** Wide QRS complex is not a direct consequence of hypercalcemia, although it can be seen in conditions that affect the ventricular depolarization, such as ventricular hypertrophy or bundle branch block.

**Option C:** Flat or inverted T waves can be seen in various conditions, including hypokalemia, myocardial infarction, and digitalis toxicity, but are not specific indicators of hypercalcemia.

**Clinical Pearl / High-Yield Fact**
It's essential to correct the QT interval for heart rate when assessing the effect of hypercalcemia on the ECG to avoid misinterpreting the results.

**Correct Answer:** C.