90% Of Dermatology
Appearance: Erythematous. Flexor surfaces and face.
Diagnosis: Clinical.
Treatment: Reassurance and moisturizer.
If it’s severe, you can prescribe topical steroids. But remember that topical steroids make the skin really thin and prone to breaking, so (if you’re feeling all fancy and private-practice-y) you could instead use a topical ointment that ends in either “-nib” or “-mab”.
If there’s systemic involvement, make sure you’re using oral corticosteroids, not topical. (That would be silly. And it would make you look silly.)
OR – or! – if your client has a lot of excess cash – you can use phototherapy! There’s no guarantee it’ll completely remove it, but it sure looks fun and is certainly very fancy and up-to-date.
9.99% of Dermatology
Appearance: Cancer.
Diagnosis: Biopsy.
Treatment: Cut it the hell out. Follow-up every 6 months.
Action Potential 
You left out the antifungal topicals. Should go in the 90% category.
KOH skin scrapings help differentiate from the reassurance/steroid category.
Or you can just give Lotrisone(combo steroid and antifungal) and skip the KOH step.
Standard disclaimer of ‘I am not your doctor and did not examine you or recommend a specific treatment apply.
I had a patient recently who developed adrenal insufficiency from prolonged topical steroid use for psoriasis. Very interesting case.
That 90% can also summarized as “if its wet, dry it. If it’s dry, wet it.” My favorite dermatologist walked into each exam room with a scapel and forceps at the ready. Oddly enough the patients found this reassuring.
I’ve actually heard Amanda’s phrase in a slightly longer expression:
If it’s dry, wet it
If it’s wet, dry it
If that doesn’t work, try steroids
If that doesn’t work, try more steroids
If that doesn’t work, biopsy it.