I believe that something sexual is considered to be a fetish when an object or a situation produces sexual arousal. But I'd say as a working definition it's rather broad since almost anything can fit into those categories. Some people need nudity to be become aroused, others manage quite well without it ...
I think it's better to talk of a fetish being a necessary precondition to sexual arousal, something that cannot be missing, something without which people do not become sexually aroused. Or also something which people do not ordinarily associate with sexual arousal, but which produces it in certain individuals - say like the classic wearing fur or high heels.
For a medical fetish I' say it's a bit of a borderline thing. Since there are lots of medical situations involving the removal of clothing or nudity, sexual elements will invariably enter into many medical situations, be it from getting an injection in the buttocks to getting a school exam or whatever.
So sometimes it's a bit fuzzy about what is or is not a fetish.