A patient is explicitly told not to shake the examiner's hand on future occasions, yet continues to shake it every single time it is offered, despite the instruction being repeated before each attempt. What is this?

This is forced grasping: the persistence of an action, here shaking an offered hand, against an explicit instruction not to, repeated across multiple offers despite the instruction remaining in place throughout, exactly as described. It is not automatic obedience, since forced grasping differs from automatic obedience in involving persistence of an action against an explicit instruction, rather than compliance with an instruction regardless of its consequences. It is not the grasp reflex, a genuinely neurological sign in which an object placed in the hand is automatically grasped, which points toward a different, more localising significance. And it is not Mitgehen, which involves movement in response to the lightest pressure rather than a persistent grasping action against explicit instruction.