Basically, John Q. Character has an intermittent tendency to dissociate physical pain, and I could use a condition that will tip him and his doctor off to this fact. My mental image involves the doctor palpating somewhere on John's body and going "Wait, doesn't that hurt? Because I can feel that your spleen is swollen (or whatever), and it should hurt when I do that."
I'm thinking something that's treatable in the short term, rather than being chronic or long-lasting. But it would also work if it fell into the "wow, it's really good we caught that early, or it would have been a lot harder to treat" category. It does have to be something where either pain is the only early symptom, or the others are relatively easy to write off as minor annoyances, not worth going to the doctor for in and of themselves.
Although, come to think of it - John recently started on an unrelated medication, and the doctor is still closely monitoring the side effects. So it would be great if I could have a scene where she says "So, John, noticed any new side effects?" and he replies "Yeah, my nails are flaking, what's up with that?" and she has that big House-esque moment of "Uh-oh! Flaking nails are a sign of swollen spleen! Even though you haven't reported abdominal pain, I'd better order an XKCD scan, just to be sure."
Any leads would be appreciated!