Final answer:
To reduce the security risk when a co-worker leaves a computer without logging off, the staff member should lock the computer screen.
Step-by-step explanation:
To reduce the security risk, the staff member should lock the computer screen when walking away from the computer without logging off. This prevents anyone from accessing the sensitive patient information. The other actions mentioned, such as evaluating the feasibility of the current policy, discussing the incident with other staff members, and requesting that staff members tell someone when leaving their computers, are all important for improving security awareness and practices, but locking the computer screen is the most immediate and effective action to reduce the risk of unauthorized access.