Contact Dermatitis: This is a common condition that can cause a darkened rash in the armpit due to irritation from deodorants, antiperspirants, or other substances that come into contact with the skin. The rash can appear as red, brown, or gray, depending on the individual's skin tone and the severity of the reaction.

Intertrigo: A skin condition characterized by inflammation of skin folds, which can include the armpits. It's often caused by friction, moisture, and bacterial or fungal infections, leading to a darkened, red, or brown rash.
Eczema (Atopic Dermatitis): Can cause a darkened rash in the armpit, especially in individuals with a history of eczema. The rash may appear as dry, scaly, or leathery patches.
Fungal Infections (e.g., Candidiasis): Fungal infections thrive in warm, moist areas like the armpits. They can cause a darkened rash, often accompanied by itching and a foul odor.

Malignancy (e.g., Breast Cancer, Lymphoma): Although rare, a darkened rash in the armpit could be a sign of an underlying malignancy, such as breast cancer or lymphoma. It's crucial to investigate any unusual or persistent changes.
Infectious Diseases (e.g., Tuberculosis, Syphilis): Certain infectious diseases can cause skin manifestations, including rashes in the armpits. These conditions require prompt diagnosis and treatment.

Addison's Disease: A rare endocrine disorder that can cause hyperpigmentation (darkening of the skin) in various areas, including the armpits.
Pityriasis Versicolor: A fungal infection that can cause discolored patches on the skin, including the armpits, although it's more commonly found on the chest and back.
Dermatologic Manifestations of Systemic Diseases (e.g., Sarcoidosis, Amyloidosis): Certain systemic diseases can have dermatologic manifestations, including rashes in unusual locations like the armpits. These conditions are rare but important to consider in the differential diagnosis.