Contact Dermatitis: This is a common condition that can cause a flat, red rash on the back, especially if the individual has come into contact with an allergen or irritant. The rash can cover a large area, including the entire back.

Eczema (Atopic Dermatitis): A chronic skin condition that can cause a red, itchy, and flat rash on the back. It often appears in individuals with a history of allergies or asthma.
Psoriasis: An autoimmune condition that can cause a red, scaly rash on the back, although the scales might not always be present. The rash can be flat and cover large areas.
Seborrheic Dermatitis: A condition that causes a red, itchy, and flaky rash, typically on oily areas of the body like the back. It can appear as a flat rash if the flakes are minimal.

Staphylococcal Scalded Skin Syndrome (SSSS): A serious condition caused by staphylococcal toxins that can lead to widespread skin peeling and redness, including the back. It's more common in children but can occur in adults, especially those with compromised immune systems.
Toxic Epidermal Necrolysis (TEN): A life-threatening skin condition usually triggered by medications, leading to widespread skin necrosis and detachment. It can start with a red, flat rash that progresses rapidly.
Lyme Disease: Caused by a tick bite, it can initially present with a distinctive "bull's-eye" rash (erythema migrans), but in some cases, the rash can be more diffuse and flat, covering large areas including the back.

Pityriasis Rubra Pilaris (PRP): A rare skin disorder that causes constant inflammation and scaling (exfoliation) of the skin. It can start with a red, flat rash on the back.
Cutaneous T-Cell Lymphoma (Mycosis Fungoides): A type of skin cancer that can cause a variety of skin symptoms, including flat, red rashes on the back. It's rare and often mimics more common skin conditions in its early stages.