**Core Concept**
Ketamine is a dissociative anesthetic agent that works by blocking NMDA receptors in the brain, leading to a disconnection between the thalamus and the cortex. It has various clinical applications, but its use is contraindicated in certain situations due to potential adverse effects.

**Why the Correct Answer is Right**
Ketamine is contraindicated in patients with a history of psychosis or schizophrenia, as it can exacerbate psychotic symptoms. This is because ketamine can mimic the effects of phencyclidine (PCP), a psychedelic drug known to cause psychotic episodes. Additionally, ketamine can increase intracranial pressure (ICP), which may be problematic in patients with head injuries or those at risk of increased ICP. However, one scenario where ketamine is not contraindicated is in patients with severe head injury, as it can provide analgesia without significant depression of respiratory or cardiovascular function.

**Why Each Wrong Option is Incorrect**
**Option A:** Ketamine is contraindicated in patients with a history of substance abuse, particularly alcohol or benzodiazepines, as it can lead to increased risk of addiction and withdrawal symptoms.

**Option B:** Ketamine is contraindicated in patients with glaucoma, as it can increase intraocular pressure, potentially exacerbating the condition.

**Option C:** Ketamine is contraindicated in patients with severe cardiovascular disease, as it can cause significant increases in blood pressure and heart rate.

**Clinical Pearl / High-Yield Fact**
When administering ketamine, it's essential to monitor patients closely for signs of increased ICP, such as headache, nausea, and vomiting. Be aware that ketamine can also cause a "ketamine-induced psychosis" in susceptible individuals, which can manifest as hallucinations, delusions, or disorganized thinking.

**Correct Answer: D. Severe head injury**