What should you know about a rash under the armpit?

Deodorant, shaving, detergent, and fabric can trigger contact dermatitis. Heat and sweat trapped in the fold may cause heat rash or intertrigo. A sharply bordered, scaly or ring-like change can be ringworm, while eczema or psoriasis may affect the fold and nearby skin. Tender lumps, pus, or warmth can indicate a follicle or skin infection and deserve assessment. Compare both armpits and note whether the rash is itchy, burning, sore, moist, cracked, or spreading.

Pause deodorant, perfume, and recently changed products. Wash gently, rinse well, pat completely dry, and choose loose clothing that reduces rubbing. Avoid shaving over irritated skin and do not share towels. Do not use a steroid, antifungal, or antibiotic cream without advice because the wrong product can mask or worsen some causes. A clinician can examine the fold and select treatment; related reading includes Contact Dermatitis, Intertrigo, Heat Rash, and Ringworm.

Seek prompt medical attention for rapidly increasing redness, severe pain, marked warmth, pus, fever, red streaks, or a new painful lump. Emergency help is appropriate for breathing difficulty or swelling of the face, lips, or throat. Book a review if the rash persists, recurs, cracks, or affects both sides after a product change.

Could this be contact dermatitis, intertrigo, heat rash, ringworm, eczema, or psoriasis? Should I stop shaving or deodorant, and which treatment is safe for this fold? What changes would suggest infection?