Intertrigo: This is a common condition in elderly patients, especially in areas with skin folds like under the breast. It's caused by friction, moisture, and bacterial or fungal infections, leading to painful, red lesions.

Eczema/Dermatitis: Contact dermatitis or atopic dermatitis could cause a red, painful lesion under the breast due to irritation from clothing, soap, or other allergens.
Pressure Ulcer: Given the patient's age, limited mobility could lead to pressure ulcers in areas under constant pressure, such as under the breast, especially if the patient spends a lot of time in bed.
Insect Bite or Sting: Though less common, an insect bite or sting could cause a painful, red lesion, especially if the patient has been outdoors or has pets that could bring in insects.

Breast Cancer: Although less likely to present as a raw red lesion, inflammatory breast cancer can cause skin changes, including redness and pain. Given the location under the breast, it's crucial not to miss this potentially deadly diagnosis.
Cellulitis: A bacterial infection of the skin that can spread rapidly and become severe, especially in elderly patients. It requires prompt antibiotic treatment.
Necrotizing Fasciitis: A rare but life-threatening infection that causes rapid destruction of the tissue under the skin. It's essential to consider this diagnosis in any case of severe, rapidly progressing skin infection.

Zinc Deficiency: Can lead to skin lesions and impaired wound healing, though this would be less common and typically associated with other systemic symptoms.
Vasculitis: Inflammation of the blood vessels can cause skin lesions, but this would usually be part of a systemic condition with other symptoms.
Pyoderma Gangrenosum: A rare skin condition that can cause painful ulcers, often in the context of systemic disease.