**Question:** Rigler's triad doesn't include

A. Pneumothorax
B. Pneumopericardium
C. Pneumomediastinum
D. Pneumoperitoneum

**Core Concept:** Rigler's triad is a set of three clinical signs associated with alveolar rupture and air-containing collections in the pleural space. These signs help differentiate between tension pneumothorax and simple pneumothorax.

**Why the Correct Answer is Right:** Rigler's triad consists of three clinical signs:
1. Pneumothorax: A true air-filled space that communicates with the pleural space, which is the correct answer.
2. Pneumopericardium: Air accumulation within the pericardial space, indicating tension pneumothorax.
3. Pneumomediastinum: Air accumulation within the mediastinum, which may indicate tension pneumothorax or a more severe condition, such as tension pneumopericardium.

**Why Each Wrong Option is Incorrect:**
1. Pneumothorax: This is a correct part of Rigler's triad but is not mentioned as a separate option.
2. Pneumopericardium: This is the second part of Rigler's triad and is associated with tension pneumothorax.
3. Pneumoperitoneum: Air accumulation within the peritoneal cavity, usually due to hollow organ rupture, such as in perforated peptic ulcer disease. It is not part of Rigler's triad as it involves abdominal organs, not the pleural space.

**Clinical Pearl:** Rigler's triad is a valuable tool in diagnosing tension pneumothorax, allowing clinicians to differentiate between simple pneumothorax and potentially life-threatening tension pneumothorax. A combination of all three signs (pneumothorax, pneumopericardium, and pneumomediastinum) strongly suggests tension pneumothorax, which requires immediate intervention.