I can tell you I can’t say any vowels. I have other letters and sounds I struggle with but vowels are by far the worst. Doesn’t matter what the word is. It could be something as simple as “a”, “I”, “is”, it doesn’t matter. I feel it get almost physically stuck in my chest area/back of my throat and I have to forcefully push it out. I can be there for a few seconds making no sound at all sometimes, so like “He…………is” and others times it could come out like “i-i-i-i-is”. It’s really annoying cause my home country and county both begin with a vowel, along with my nationality so telling people where I’m from is really hard. Also the “Eu” and the start of Europe is the hardest sound I’ve come across. No matter how many times I come across it I will always stutter ok it I also struggle to say my name which is very common in people who stutter. It could take me ages just to say my own name so that’s not too fun. Just something to add. Hope it helps