A client with obsessive-compulsive disorder washes her hands excessively. What defense mechanism is she using?

The defense mechanism demonstrated by the client who excessively washes her hands is best identified as undoing. Undoing is a psychological defense mechanism in which an individual tries to counteract or negate feelings or thoughts that provoke anxiety through actions that symbolize the opposite. In this case, the act of hand washing serves as a ritualistic behavior to alleviate or counteract feelings of guilt, anxiety, or discomfort associated with intrusive thoughts or obsessions. By washing her hands excessively, the client attempts to "undo" her thoughts or feelings that she may perceive as negative or contaminated.

This behavior is typical in individuals with obsessive-compulsive disorder, where compulsive actions are performed in hopes of mitigating the anxiety produced by obsessive thoughts. The hand washing is a concrete action that the client believes can reduce distress or prevent a feared consequence, reflecting the underlying dynamics of undoing.