Typically "a" or "an" are used when the word could potentially be plural. You're indicating that it's only one thing. So in the second sentence, it would not make sense to say he gives many attentions to students; thus you don't indicate he only gives "an attention," In your first sentence, it is technically something that could be plural (such as he has two genetic defects, one of which makes him unable to think clearly). In the third sentence, the "an" is referring to the state, not the individual. National Guards are usually called to a single state, so they are indicating that it's not something that typically occurs across multiple states at once.