## Core Concept
Tall T waves on an electrocardiogram (ECG) are a significant finding often associated with **hyperkalemia**, a condition characterized by elevated potassium levels in the blood. This ECG change is one of the earliest signs of hyperkalemia. The T wave represents the **repolarization** of the ventricles.

## Why the Correct Answer is Right
The correct answer, **hyperkalemia**, is associated with tall, peaked T waves on an ECG. This occurs because high potassium levels affect the **resting membrane potential** and **action potential** of cardiac cells, leading to changes in the ECG. Specifically, hyperkalemia causes the T waves to become tall and peaked, often described as "tented." This is due to the rapid repolarization phase.

## Why Each Wrong Option is Incorrect
- **Option A:** Hypokalemia is characterized by low potassium levels and typically leads to **flattened T waves**, **ST segment depression**, and **U waves** on an ECG, not tall T waves.
- **Option B:** Hypocalcemia (low calcium levels) primarily affects the **QT interval**, prolonging it, but does not directly cause tall T waves.
- **Option C:** Hypercalcemia (high calcium levels) can also affect the **QT interval**, shortening it, but is not primarily associated with tall T waves.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that tall T waves are an **early sign of hyperkalemia**. Recognizing this on an ECG can be life-saving, as hyperkalemia can quickly progress to more severe cardiac arrhythmias and cardiac arrest if not promptly treated.

## Correct Answer Line
**Correct Answer: D. Hyperkalemia**