Korsakoff syndrome is the chronic, irreversible phase of the Wernicke-Korsakoff spectrum, driven by depletion of thiamine ($B_1$) and damage to the mammillary bodies and dorsomedial thalamic nuclei. The cardinal cognitive defect is a failure of new learning: information presented after the onset cannot be consolidated into long-term storage.
This forward-facing memory failure is termed anterograde amnesia. Clinically the patient greets the examiner repeatedly, forgets instructions within minutes, and bridges the resulting gaps with confabulation. Immediate (working) memory span and previously learned skills stay largely intact, distinguishing it from a global dementia.
Contrast this with retrograde amnesia (loss of pre-existing memories), transient global amnesia (a brief reversible event), and dissociative amnesia (a psychological, non-organic loss of personal history). None of these match the thiamine-related learning block.
\[\boxed{\text{Korsakoff syndrome} \rightarrow \text{anterograde amnesia}}\]