**Core Concept**
A left pneumothorax is a condition where air accumulates in the pleural space adjacent to the left lung, leading to increased intrathoracic pressure and subsequent shift of the mediastinum to the right. This can cause various ECG changes due to the mechanical compression and displacement of the heart.

**Why the Correct Answer is Right**
In a left pneumothorax, the ECG changes are primarily due to the mechanical compression of the heart. The correct answer is **A. Left axis deviation**. This is because the increased intrathoracic pressure and mediastinal shift to the right can compress the left side of the heart, leading to left axis deviation. The left axis deviation is a result of the altered electrical axis of the heart due to the mechanical compression.

**Why Each Wrong Option is Incorrect**
* **Option B:** Right axis deviation is more likely in a right pneumothorax, not a left pneumothorax.
* **Option C:** Prolonged QT interval is not a typical ECG change in pneumothorax.
* **Option D:** The ECG changes in pneumothorax are primarily mechanical and not related to electrolyte imbalances.

**Clinical Pearl / High-Yield Fact**
In a patient with pneumothorax, the ECG changes can be subtle, and a chest X-ray is often necessary to confirm the diagnosis.

**Correct Answer: B. Right axis deviation**