Atopic Dermatitis (Eczema): This is a common condition characterized by itchy, inflamed skin that can appear all over the body. It's often associated with allergies and can be triggered by various factors including environmental allergens, stress, and irritants.

Contact Dermatitis: This condition occurs when the skin comes into contact with an allergen or irritant, leading to a rash. It can appear anywhere on the body and is common in individuals who work with chemicals or have frequent exposure to potential allergens.
Urticaria (Hives): Characterized by itchy, raised welts, urticaria can be caused by allergic reactions to foods, medications, or infections. It can appear all over the body and may be accompanied by other symptoms like swelling.
Drug Rash: Many medications can cause skin rashes as a side effect. These rashes can vary in appearance and can cover large areas of the body. Identifying and stopping the offending drug usually resolves the rash.

Stevens-Johnson Syndrome (SJS) and Toxic Epidermal Necrolysis (TEN): These are severe skin conditions usually triggered by medications. They can start with a rash that progresses to blistering and peeling of the skin, involving the mucous membranes. Early recognition is crucial as these conditions can be life-threatening.
Anaphylaxis: Although primarily known for its respiratory and cardiovascular symptoms, anaphylaxis can also present with a widespread rash. It's a medical emergency requiring immediate treatment.
Severe Cutaneous Adverse Reactions (SCARs): This category includes conditions like Drug Reaction with Eosinophilia and Systemic Symptoms (DRESS) syndrome, which can present with a rash, fever, and involvement of internal organs. These conditions are rare but potentially life-threatening.

Pemphigus: A group of autoimmune diseases that cause blistering of the skin and mucous membranes. While rare, pemphigus can present with a widespread rash and is important to diagnose early due to its potential severity.
Erythema Multiforme: An immune-mediated disorder that can cause skin lesions, often triggered by infections or medications. It's relatively rare and can have a varied presentation, including a rash all over the body.
Cutaneous T-cell Lymphoma (CTCL): A type of cancer that affects the skin, CTCL can cause a variety of skin lesions, including rashes. It's rare and diagnosis often requires a skin biopsy.