**Core Concept**
Hypokalemia is characterized by a deficiency of potassium ions (K+) in the body, leading to various cardiac and neuromuscular manifestations. The ECG changes associated with hypokalemia are crucial for its early detection and management.

**Why the Correct Answer is Right**
In hypokalemia, the ECG may show a characteristic pattern of ST-segment depression, T-wave flattening or inversion, and U-wave prominence. This occurs due to the reduced potassium ion concentration affecting the cardiac action potential, particularly the plateau phase. The reduced potassium ions lead to a decrease in the duration of the action potential, resulting in a shorter QT interval.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because hyperkalemia (elevated potassium levels) typically presents with peaked T-waves, widened QRS complex, and a prolonged QT interval, not ST-segment depression.
**Option B:** This option is incorrect because hypercalcemia (elevated calcium levels) primarily affects the cardiac conduction system, causing shortened QT interval, widened QRS complex, and increased T-wave amplitude.
**Option C:** This option is incorrect because hypernatremia (elevated sodium levels) does not have a direct effect on the ECG, and its effects are more related to the volume status and osmolality of the body.

**Clinical Pearl / High-Yield Fact**
In patients with hypokalemia, the ECG changes can be a harbinger of life-threatening cardiac arrhythmias. It is essential to promptly identify and correct hypokalemia to prevent these complications.

**Correct Answer:** D. Hypokalemia.