**Core Concept**
Hypokalaemia is a condition characterized by low serum potassium levels, which can lead to various cardiac, muscular, and gastrointestinal symptoms. The body's potassium balance is tightly regulated by the kidneys, and hypokalaemia can result from excessive potassium loss, inadequate potassium intake, or impaired renal function.

**Why the Correct Answer is Right**
The correct answer is related to the pathophysiological effects of hypokalaemia on the heart. In hypokalaemia, the electrocardiogram (ECG) may show **U waves**, which are an important diagnostic feature. U waves are thought to represent **late phase 3 of the cardiac action potential**. Hypokalaemia can also lead to **prolongation of the QT interval**, which increases the risk of **torsades de pointes**, a life-threatening arrhythmia.

**Why Each Wrong Option is Incorrect**
* **Option A:** Muscle weakness is a common symptom of hypokalaemia, so this option is incorrect because it is associated with the condition.
* **Option B:** Gastrointestinal symptoms like nausea and vomiting can occur in hypokalaemia, making this option incorrect as well.
* **Option C:** This option is incorrect because hypokalaemia can lead to an increased risk of cardiac arrhythmias, including ventricular fibrillation.

**Clinical Pearl / High-Yield Fact**
One key point to remember is that hypokalaemia can lead to **cardiac arrest**, particularly in severe cases. It is essential to identify and manage hypokalaemia promptly to prevent serious complications.

**Correct Answer: D.**