**Core Concept**
A left pneumothorax can lead to various ECG changes due to the displacement of the heart and mediastinal structures. The underlying principle is that the heart is shifted away from the affected side, affecting the electrical activity of the heart.

**Why the Correct Answer is Right**
In a left pneumothorax, the heart is displaced to the right, which can lead to a decrease in the amplitude of the P wave (due to reduced atrial depolarization), a decrease in the amplitude of the QRS complex (due to reduced ventricular depolarization), and a decrease in the PR interval (due to the altered electrical pathway). However, the ECG change least likely to occur is an increase in the QRS voltage, as the heart is displaced away from the electrodes, reducing the electrical activity recorded.

**Why Each Wrong Option is Incorrect**
**Option A:** This option might be a distractor, but it's not the least likely change. A decrease in the P wave amplitude is a plausible change due to reduced atrial depolarization.
**Option B:** While a decrease in the PR interval can occur, it's not the least likely change. The altered electrical pathway can indeed affect the PR interval.
**Option C:** This option is likely a distractor, but it's not the correct answer. An increase in the QRS voltage is actually more likely to occur in a right pneumothorax, not a left one.

**Clinical Pearl / High-Yield Fact**
In cases of pneumothorax, the ECG changes can be subtle and may not always indicate the presence of a pneumothorax. It's essential to consider the clinical context and other diagnostic findings to make an accurate diagnosis.

**Correct Answer: C.**