**Core Concept**
Korsakoff's psychosis, also known as Korsakoff's syndrome, is a neurological disorder characterized by profound memory impairments, particularly in the formation of new memories (anterograde amnesia). This condition is often associated with chronic alcohol abuse and malnutrition, which lead to thiamine deficiency.

**Why the Correct Answer is Right**
The correct statement about Korsakoff's psychosis is that it is characterized by severe anterograde memory defect, which refers to the inability to form new memories after the onset of the disorder. In addition to anterograde amnesia, patients with Korsakoff's psychosis often exhibit mild retrograde memory defect, which refers to a loss of memories formed before the onset of the disorder. This is because the hippocampus and other parts of the temporal lobe, which are responsible for memory consolidation, are severely affected in Korsakoff's psychosis.

**Why Each Wrong Option is Incorrect**
**Option B:** Mild anterograde + severe retrograde memory defect. This is incorrect because Korsakoff's psychosis is typically characterized by severe anterograde amnesia, not mild.

**Option C:** Only antegrade memory defect. This is incorrect because while anterograde amnesia is a hallmark of Korsakoff's psychosis, patients may also exhibit some degree of retrograde amnesia.

**Option D:** Only retrograde memory defect. This is incorrect because Korsakoff's psychosis is primarily characterized by anterograde amnesia, not retrograde amnesia.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that Korsakoff's psychosis is a treatable condition, and prompt administration of thiamine supplements can help alleviate symptoms and improve memory function. However, delayed treatment can lead to irreversible cognitive decline.

**✓ Correct Answer: A. Severe antegrade + Mild retrograde memory defect**