Differential Diagnosis

• Single most likely diagnosis
  • Atopic Dermatitis (Eczema): The patient's history of childhood eczema, severe exacerbation over the past 6 months, and description of "unbearable" itching are consistent with atopic dermatitis. The variation in rash appearance on different body parts and the involvement of typical areas such as the arms, face, chest, armpit area, back, and back of knees also support this diagnosis.
• Other Likely diagnoses
  • Contact Dermatitis: The patient's symptoms could be exacerbated by contact with irritants or allergens, especially given the distribution of the rash and the severe itching.
  • Psoriasis: Although less common, psoriasis could present with severe itching and a rash, particularly on the elbows, knees, and back, which overlaps with some of the patient's symptoms.
  • Seborrheic Dermatitis: This condition can cause a rash and itching, particularly on the face, chest, and back, and could be considered given the patient's symptoms.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
  • Cutaneous T-Cell Lymphoma (Mycosis Fungoides): Although rare, this condition can present with severe itching and skin lesions, and if missed, can have a poor prognosis.
  • Bullous Pemphigoid: An autoimmune blistering disease that can cause severe itching and skin lesions, which could be considered in the differential diagnosis due to its potential severity.
• Rare diagnoses
  • Dermatitis Herpetiformis: A rare skin condition characterized by intensely itchy blisters, which could be considered given the patient's severe itching, although the rash description does not fully align.
  • Erythroderma (Exfoliative Dermatitis): A condition characterized by widespread redness and scaling of the skin, which could be a rare consideration given the patient's extensive rash and severe symptoms.