Differential Diagnosis for Rash

The following is a differential diagnosis for various types of rash, categorized for clarity and emphasis on critical conditions.

• Single Most Likely Diagnosis

  • Eczema (Atopic Dermatitis): A common condition characterized by itchy, inflamed skin, often related to allergies or asthma.
  • Contact Dermatitis: An allergic or irritant reaction to substances like metals, soaps, or plants, leading to red, itchy skin.
  • Viral Exanthem: A rash caused by viral infections, such as measles, chickenpox, or roseola, often accompanied by fever and other systemic symptoms.

• Other Likely Diagnoses

  • Psoriasis: An autoimmune condition leading to thick, scaly, red patches on the skin, often on the elbows, knees, or scalp.
  • Allergic Reactions: Including urticaria (hives) and angioedema, which can be caused by food, drugs, or insect bites.
  • Fungal Infections: Such as ringworm (tinea corporis), athlete's foot (tinea pedis), or jock itch (tinea cruris), typically causing circular, itchy lesions.
  • Bacterial Infections: Like impetigo, a contagious infection of the skin causing red sores, or folliculitis, an infection of the hair follicles.

• Do Not Miss Diagnoses

  • Meningococcemia: A severe infection caused by Neisseria meningitidis, which can lead to a rash (purpura) and is life-threatening if not promptly treated.
  • Rocky Mountain Spotted Fever (RMSF): A tick-borne disease that can cause a rash, fever, and severe illness if not treated early.
  • Stevens-Johnson Syndrome/Toxic Epidermal Necrolysis (SJS/TEN): Severe skin and mucous membrane disorders, usually a reaction to medication or infection, which can be fatal.
  • Lyme Disease: Caused by the bacterium Borrelia burgdorferi, transmitted by ticks, and can cause a distinctive "bull's-eye" rash, along with other symptoms.

• Rare Diagnoses

  • Pityriasis Rubra Pilaris: A rare skin disorder causing constant inflammation and scaling (exfoliation) of the skin.
  • Kawasaki Disease: An illness that affects children, causing fever, rash, and swelling, with potential for serious cardiac complications.
  • Erythema Multiforme: An immune-mediated disorder characterized by target-like lesions on the skin, often in response to infections or medications.
  • Sweet Syndrome (Acute Febrile Neutrophilic Dermatosis): A rare condition marked by fever, leukocytosis, and tender, erythematous skin lesions.