MRCP2-3365

A 55-year-old man comes to the clinic with complaints of difficulty walking and disorientation. He also reports experiencing blurred and double vision. Upon examination, he displays severe vertical and horizontal nystagmus and depression of the deep tendon reflexes. The patient rarely visits his GP and is not taking any medication. He is a heavy smoker, consuming 60 cigarettes a day, and drinks a bottle of cider daily. What vitamin deficiency is associated with this condition and requires replacement?