bald spot on cat

Question
Hello, my name is Katie.  I have a 5-6 yr old long haired cat named Mulder. He's always been an indoor cat, and I only have one other pet, an 18 yr old cat who is also indoor only.
I just noticed that Mulder has a pretty sizable bald spot on his back, right in the middle. This is a spot he can't easily get to so I don't think over grooming is an issue. He doesn't seem to be irritated by it and the spot looks normal (no redness, puffiness, irritation etc). I did change his food recently (about a week and a half ago) from blue buffalo grain free to purina pro plan protein plus i think it's called. Could that be an issue?

Also, he was doing really well on the blue buffalo, I only changed because a friend of mine said that his cat had to get bladder surgery and he said that he heard blue buffalo was linked to bladder/urinary issues. Or maybe it's because he's no longer on grain free food?
I would really appreciate any advice you can give on this because if at all possible I'd like to avoid spending $50 on a vet visit just to ask them some questions. I hope I've provided you with enough information. Thank you in advance.

Answer
When I see a single area of baldness on a cat I think of one of three possibilities to rule out.  One is a vaccine reaction.  Sometimes a vasculitis will develop at a vaccination site causing atrophy of the hair follicles.  Also any injectable steroids given under the skin can cause the hair follicles to stop producing hairs.  And there is always the possibility of ringworm fungi to cause a circular patch of hair loss.  If the hair does not grow back, then you might need to see your vet, but it can probably wait as long as the cat is asymptomatic.