**Core Concept**
Reversible causes of dementia refer to conditions that can lead to cognitive decline, but can be effectively treated or managed, thereby reversing symptoms. These conditions often have a distinct underlying pathophysiology, and prompt diagnosis and treatment are crucial to prevent long-term damage.

**Why the Correct Answer is Right**
The correct options are **a) Hypothyroidism** and **c) Vitamin B12 deficiency**. Hypothyroidism is a condition where the thyroid gland does not produce enough thyroid hormones, which are essential for brain function. Thyroid hormone deficiency can lead to cognitive impairment, depression, and fatigue. Treatment with synthetic thyroid hormones can reverse these symptoms. Vitamin B12 deficiency is another reversible cause of dementia, as it affects the synthesis of myelin, the fatty substance that surrounds and protects nerve fibers. A deficiency in Vitamin B12 can lead to neurological symptoms, including cognitive decline, weakness, and numbness. Treatment with Vitamin B12 supplements can reverse these symptoms.

**Why Each Wrong Option is Incorrect**
**Option B:** Alzheimer's disease is the most common cause of irreversible dementia. It is a progressive neurodegenerative disorder characterized by the accumulation of amyloid-beta plaques and tau protein tangles in the brain, leading to neuronal death and cognitive decline. While there are some treatments available to manage symptoms, there is currently no cure for Alzheimer's disease.
**Option D:** Vitamin A deficiency is not a known cause of dementia. While Vitamin A is essential for maintaining healthy vision, immune function, and skin health, it does not play a significant role in cognitive function or the development of dementia.

**Clinical Pearl / High-Yield Fact**
When evaluating patients with dementia, it is essential to consider reversible causes, such as hypothyroidism, Vitamin B12 deficiency, and medication side effects. A thorough medical history, physical examination, and laboratory tests can help identify these conditions and guide treatment.

**✓ Correct Answer: A. ac. Both hypothyroidism and Vitamin B12 deficiency are reversible causes of dementia.**