Question

A 58-year-old man is brought to the clinic by his husband due to a 14-month history of progressive behavioral changes. Previously known as a meticulous and empathetic high school teacher, the patient has become increasingly apathetic and frequently makes sexually explicit remarks to neighbors. He recently ate a stranger's sandwich off their plate at a local restaurant and showed no remorse when confronted by the staff. He denies feeling sad or anxious, stating, "Everything is perfectly fine." The patient has a remote history of a single depressive episode in his 20s. He takes over-the-counter diphenhydramine occasionally for sleep but takes no prescription medications. Vital signs are within normal limits. Laboratory studies, including a thyroid-stimulating hormone (TSH) level of 1.2 mU/L and a free T4 of 1.2 ng/dL (normal: 0.8-1.8 ng/dL), are normal. On mental status examination, he has a blunted affect and poor insight, but no delusions or hallucinations are elicited. Montreal Cognitive Assessment (MoCA) score is 23/30; he loses points primarily on clock drawing and verbal fluency tasks, but successfully recalls 3 out of 3 objects after a 5-minute delay. Neurological examination reveals no focal weakness, rigidity, or resting tremor. Which of the following is the most likely diagnosis?