Icupphobia
If fear of words that sound like single letters was real this would be it's name. It comes from spell icup
kid 1: Spell icup
kid 2: Sure, I see AAAAAAAAAAAAAAAAAAAAAAAAA
kid 1: I think he has icupphobia
kid 2: Sure, I see AAAAAAAAAAAAAAAAAAAAAAAAA
kid 1: I think he has icupphobia