While looking at an ordinary traffic light change from red to green, a patient suddenly becomes completely convinced that this means he must abandon his family immediately, a belief he holds with total, unshakeable conviction from that moment on. Before revealing the answer: is this an autochthonous delusion?

No, this is more consistent with a delusional perception than with an autochthonous delusion. Here, a real, external perceptual event, the traffic light changing, is present, and the new delusional meaning is attached specifically to that genuine perception. An autochthonous delusion would instead involve the belief appearing with no such external perceptual trigger at all, arising directly and completely in the patient's mind, which is not the case in this vignette.