**Core Concept**
Reversible dementia refers to a condition where cognitive decline can be reversed or improved with treatment, in contrast to irreversible dementias like Alzheimer's disease. This concept is crucial in psychiatry, as timely diagnosis and intervention can significantly impact patient outcomes.

**Why the Correct Answer is Right**
Alzheimer's disease is the most common cause of irreversible dementia, characterized by progressive neuronal loss and amyloid-beta plaques in the brain. In contrast, reversible dementias are caused by treatable conditions, such as nutrient deficiencies, infections, or metabolic disorders. Alzheimer's disease is not reversible with current medical interventions, and its diagnosis is typically made based on clinical presentation, laboratory tests, and imaging studies.

**Why Each Wrong Option is Incorrect**
**Option A:** Wernicke's encephalopathy is a reversible condition caused by thiamine deficiency, often seen in chronic alcoholics. It can lead to cognitive impairment, ataxia, and ophthalmoplegia, but prompt treatment with thiamine can reverse the symptoms.

**Option C:** Hypothyroidism can cause cognitive impairment, memory loss, and depression, but these symptoms can be reversed with thyroid hormone replacement therapy. Therefore, hypothyroidism is a reversible cause of dementia.

**Option D:** Head trauma can cause cognitive decline, including dementia, especially in cases of severe traumatic brain injury. Some of these symptoms may be reversible with rehabilitation and treatment, although the extent of recovery varies widely.

**Clinical Pearl / High-Yield Fact**
Reversible dementias often present with a subacute onset, and a thorough medical history, physical examination, and laboratory tests can help identify the underlying cause. It's essential to consider reversible causes of dementia in patients with new-onset cognitive decline, as prompt treatment can significantly improve outcomes.

**✓ Correct Answer: B. Alzheimer's**