A 50-year-old male presents to the emergency department with persistent confusion over the past 3 weeks. He has a known history of HIV and has been compliant with his medications since his diagnosis 4 years ago. His past medical history includes outpatient treatment for lymphogranuloma venereum and type 2 diabetes mellitus. On examination, he is disoriented in time and place and scores 0/10 on the abbreviated mental test. A mild early diastolic murmur is heard on cardiac examination. Neurological examination reveals absent reflexes in both lower limbs, with an upgoing plantar on the left and withdrawn plantar on the right. Erythematous soles are noted on both feet. Blood tests and blood glucose are pending. What is the most likely diagnosis?