Hello!
These different areas could be due to different causes. It is possible that a single cause could be related but would seem an unusual distribution. The rash under the breast is very common for a yeast type of skin infection to occur called candidal dermatitis. This area is warm, moist and dark and these conditions tend to benefit the development of a yeast type of infection. You said, however that the rash was not red but was itchy and candidal dermatitis tends to be both. Another type of fungal infection is a tinea type of infection which is also fungal in nature but tends to have different characteristics. This type of rash is less likely to be red, it is itchy and may be a bit scaly in appearance. This type of rash would respond well to OTC Lamisil among other OTC anti-fungals. The rash under the arms could be a sensitivity to deoderant. That is one common scenario that we see in this area and this would be a contact dermatitis or atopic dermatitis. If this were the cause then avoiding using the irritating product might help as well as using a topical steroid. The rash behind the knees could be eczema which is a common hypersensitivity type skin reaction that is more common in cold dry conditions.If you had scattered dry raised firm redish brown bumps in all three areas that itched intensely then scabies might be the culprit. In order to get a definite diagnosis you would probably be best served by seeing a dermatologist to check these regions out. I hope the info has been helpful. Follow up questions are welcome if you have any.
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Anthony Bray MD
Doctor
14 years as clinician in the field of Family Practice