**Core Concept**
The presence of gas under the diaphragm on an X-ray is indicative of free air in the abdominal cavity, typically resulting from a perforated viscus (a hollow organ). This condition requires prompt surgical intervention to prevent further complications such as peritonitis and sepsis.

**Why the Correct Answer is Right**
In the context of a penetrating knife injury to the abdomen, the most likely cause of free air is a perforated intestine. The intestine is a hollow viscus that can easily become compromised due to trauma, leading to leakage of intestinal contents into the abdominal cavity. This leakage can cause the formation of gas bubbles, which accumulate under the diaphragm and are visible on an X-ray. The intestine is more susceptible to damage from penetrating injuries compared to solid organs like the spleen, liver, or lung.

**Why Each Wrong Option is Incorrect**
**Option A:** The spleen is a solid organ that is more likely to be damaged by blunt trauma, such as a motor vehicle accident, rather than a penetrating injury like a knife stab.

**Option C:** The liver is also a solid organ that can be damaged by penetrating injuries, but it is less likely to cause free air in the abdominal cavity due to its solid nature.

**Option D:** The lung is not located in the abdominal cavity, so it is not possible for a penetrating injury to the abdomen to cause free air in the lung.

**Clinical Pearl / High-Yield Fact**
When interpreting X-rays in patients with abdominal trauma, remember that free air under the diaphragm is a classic sign of a perforated viscus, and prompt surgical intervention is required to prevent further complications.

✓ Correct Answer: B. Intestine