When an external touch screen exhibits flickering without registering input on a presentation laptop, the first troubleshooting step should be to check the cables connecting the touch screen to the laptop. This includes ensuring that the cables are securely connected, undamaged, and are the correct type for transmitting both video and touch input data. Faulty, loose, or incorrect cabling can lead to issues like flickering and unregistered inputs. Reference: CompTIA A+ Core 1 (220-1101) Exam Objectives, particularly the sections dealing with peripheral devices and troubleshooting connectivity and performance issues related to external devices like touch screens. Basic troubleshooting principles for peripheral devices, emphasizing the importance of verifying physical connections and cable integrity when encountering issues with device functionality.