**Core Concept**
A small pneumothorax is a condition where there is a minimal accumulation of air in the pleural space surrounding the lungs. This can be challenging to diagnose, as it may not be apparent on standard chest radiographs. The correct imaging modality for detecting small pneumothoraces is crucial for early diagnosis and management.

**Why the Correct Answer is Right**
The correct answer is **C. Chest CT scan with contrast**. A chest CT scan with contrast is highly sensitive for detecting small pneumothoraces, as it can visualize the air leak and the pleural space with high resolution. The contrast agent helps to distinguish the lung parenchyma from the pleural space, making it easier to detect small pneumothoraces. Additionally, CT scans can be performed quickly and are widely available.

**Why Each Wrong Option is Incorrect**
* **Option A:** Chest X-ray is not sensitive enough to detect small pneumothoraces, as it may not show the air leak until it becomes significant.
* **Option B:** Ultrasound is not typically used for detecting pneumothoraces, as it is not as sensitive as CT scans for this purpose.
* **Option D:** MRI is not typically used for detecting pneumothoraces, as it is not as sensitive as CT scans for this purpose and may not be available in all settings.

**Clinical Pearl / High-Yield Fact**
Remember that a small pneumothorax may not be immediately apparent on chest X-ray, and a high index of suspicion is necessary for early diagnosis. A chest CT scan with contrast should be considered if there is a high suspicion of pneumothorax, especially in patients with respiratory distress or hypoxia.

**Correct Answer:** C. Chest CT scan with contrast