The history of cardiac surgery 2 days prior followed by behavioral changes is suggestive of delirium. The question here is stressing on "disturbances of memory" which can be seen in delirium, however are usually restricted to sho term memory loss. The other impoant features such as clouding of consciousness and attention impairment have not been provided. Nonetheless, the most likely diagnosis appears to be delirium. As delirium has prominent cognitive dysfunction, that is the correct answer. Alzheimer disease does not have such sudden onset.