**Core Concept**
Gastric lavage is a procedure used to remove contents from the stomach, often in cases of poisoning or ingestion of toxic substances. However, its use is limited due to potential complications and contraindications. The procedure involves introducing a tube into the stomach to flush out its contents, which can be hazardous in certain situations.

**Why the Correct Answer is Right**
The correct answer is **C. Comatose or unconscious patients**. Gastric lavage is contraindicated in comatose or unconscious patients because they are unable to protect their airway, and the procedure can lead to aspiration of stomach contents into the lungs, causing severe respiratory complications. This is a significant concern, as these patients are already at risk due to their altered mental status. Additionally, the procedure can cause further distress and discomfort, which may worsen their condition.

**Why Each Wrong Option is Incorrect**
* **Option A:** Patients with full stomachs may not be ideal candidates for gastric lavage, but it's not the primary contraindication. Gastric lavage can still be performed with caution in these cases.
* **Option B:** Patients with suspected esophageal or gastric rupture are not the primary contraindications for gastric lavage. However, these conditions may require emergency intervention, and gastric lavage may not be the immediate priority.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that gastric lavage is generally contraindicated in patients who are unable to protect their airway, such as those who are comatose or unconscious. This is a critical consideration, as aspiration of stomach contents can lead to severe respiratory complications.

**Correct Answer: C. Comatose or unconscious patients**