**Core Concept**
Hyperkalemia is a condition characterized by elevated levels of potassium ions (K+) in the bloodstream, which can disrupt normal cardiac function. The initial ECG changes in hyperkalemia are critical for early detection and management.

**Why the Correct Answer is Right**
The initial ECG change in hyperkalemia is the appearance of tall, tented T waves. This occurs due to the effects of potassium ions on the cardiac action potential. Potassium ions play a crucial role in the repolarization phase of the cardiac cycle, and elevated levels of K+ can lead to premature repolarization, resulting in the characteristic tall, tented T waves. This change is often seen before other ECG abnormalities, such as PR prolongation or QRS widening.

**Why Each Wrong Option is Incorrect**
**Option B:** PR prolongation is a later ECG change in hyperkalemia, typically seen as potassium levels continue to rise. It is not the initial change.
**Option C:** QRS widening is another later ECG change in hyperkalemia, often seen as potassium levels approach 6-7 mmol/L. It is not the initial change.
**Option D:** ST segment depression is not a characteristic ECG change in hyperkalemia. In fact, the ST segment often becomes elevated in hyperkalemia.

**Clinical Pearl / High-Yield Fact**
The "tall, tented T waves" associated with hyperkalemia can be remembered using the mnemonic "Tall, Tentative, and Tented" to help distinguish this early change from other ECG abnormalities.

**✓ Correct Answer: A. Tall tented T waves**