**Core Concept**
The presence of gas under the diaphragm on a chest X-ray is often indicative of a pneumoperitoneum, which is the accumulation of free air within the abdominal cavity. This can occur due to a perforation of the gastrointestinal tract, typically in the setting of a peptic ulcer or intestinal obstruction.

**Why the Correct Answer is Right**
The correct answer is related to the clinical condition where gas under the diaphragm is not typically seen. One such condition is a pericardial effusion with cardiac tamponade. In this scenario, the increased pressure within the pericardial sac can push the heart downward, causing the diaphragm to be elevated, but the presence of gas under the diaphragm is not a characteristic finding. This is because the air is usually confined to the pericardial space, not the abdominal cavity.

**Why Each Wrong Option is Incorrect**
* **Option A:** Typically, a pneumoperitoneum is a condition where gas under the diaphragm is seen, making it an incorrect answer.
* **Option B:** A pulmonary embolism may cause an elevation of the diaphragm due to atelectasis or pleural effusion but does not directly relate to the absence of gas under the diaphragm.
* **Option D:** A hiatal hernia may cause air to enter the mediastinum, but it is not a condition where gas under the diaphragm is not seen.

**Clinical Pearl / High-Yield Fact**
In a patient with suspected abdominal trauma, the presence of free air under the diaphragm on a chest X-ray is a critical finding that may indicate a need for immediate surgical intervention.

**Correct Answer:** C.