A lot of people seemed to miss the part where this is a hypothetical situation. I'll answer seriously...sort of. IF I had all A's except for one B I definitely don't, btw) and an adcom asked me about it, there would be 3 options for a response.
1) The serious response: Explain how something different was going on in your life, the subject was just especially tough for you, you had a horrible lecturer for a professor who tested only on his/her own lecture material rather than the text, you had to take the class at a time slot you're not used to, you just started a new job, etc. Just anything that was different about the class that could explain that difference.
2) The humor response: Make a joke about it. I'm better at situational humor than pre-written jokes so I have no examples for you, but you've seen plenty in the other posts here. This could be good, because you don't want to seem too stiff and serious.
3) The BS approach: This would be similar to the serious response, but slightly exaggerated. You could mention something that was indeed going on while you took the B course which could explain the slight grade decline, even though you know for a fact that it didn't hinder your performance. If you aren't good at having people believe you when you're BS or stretching the truth, I wouldn't try this.
You could also lead in with a joke, then move into the serious answer. This is basically how it is for any odd low grade explanation. The exception would be for lower grades like D's or frequent C's and B's where you might want to skip the humor altogether since you probably don't have the room for screwing around. A 3.97 GPA student probably can afford to throw in some charm.
