Psoriasis: This is a common skin condition characterized by itchy, scaly patches, often appearing on the lower back and arms. The sudden appearance of these symptoms aligns with a potential psoriatic flare.

Eczema (Atopic Dermatitis): Eczema can cause itchy, scaly patches and is common on the arms. While it often starts in childhood, it can persist into adulthood or have late onset.
Seborrheic Dermatitis: This condition causes scaly patches, red skin, and stubborn dandruff, often on oily areas of the body like the upper back, but can also appear on the arms.
Contact Dermatitis: An allergic reaction or irritation from substances like soaps, detergents, or metals can cause itchy, scaly patches on exposed areas, including the arms and potentially the lower back.

Syphilis (Secondary): Although less common, secondary syphilis can cause a rash on the palms of the hands and soles of the feet, but it can also appear on other parts of the body, including the back and arms. It's crucial to consider this due to its serious implications if left untreated.
Lymphoma (Cutaneous T-Cell Lymphoma): Certain types of lymphoma can initially present with skin symptoms, including itchy patches. While rare, missing this diagnosis could have severe consequences.

Pityriasis Rubra Pilaris: A rare skin disorder that causes constant itching and scaling (exfoliation) of the skin, often starting on the face and spreading to other parts of the body.
Keratosis Pilaris: Characterized by small, rough, sandpapery bumps on the skin, often on the arms, but can be itchy and might be considered in the differential for itchy, scaly patches, though it's less likely to present suddenly.
Dermatomyositis: An inflammatory disease characterized by skin rash and muscle weakness, which can include itchy, scaly patches, but is much rarer and typically involves other systemic symptoms.