not sure if I understand your idea, but you could try doing this:
prompt = ['does AN fibre',fibres_without_BF{i},'look ok?']; % create flexible prompt
ok = input(prompt, 's'); % this is the correct syntax
simply, if you want your prompt (question) to change by for loop, you should create the prompt and then just call it inside input function.
edit: another version
prompt = sprintf('does AN fibre %s look ok?',fibres_without_BF{i});
hope it helps ÷)
