Differential Diagnosis

The patient's presentation of a diffuse, intensely pruritic rash that began on her feet and progressed to other parts of her body, along with symptoms such as severe itching, swelling, and bumpy lesions, suggests several potential diagnoses. The recent outdoor exposure and use of new sunscreen are also important factors to consider.

• Single Most Likely Diagnosis

  • Contact Dermatitis: This is the most likely diagnosis given the patient's recent outdoor exposure, use of new sunscreen, and the initial involvement of her feet and subsequent spread to other areas. The intense pruritus, swelling, and bumpy lesions are consistent with an allergic contact dermatitis reaction, possibly to an ingredient in the Banana Boat sunscreen or another substance she came into contact with outdoors.

• Other Likely Diagnoses

  • Atopic Dermatitis (Eczema): Although the patient has no significant past medical history, the presentation of intense pruritus and the distribution of the rash could suggest atopic dermatitis, especially if she has a personal or family history of atopy that was not mentioned.
  • Irritant Contact Dermatitis: This could be considered if the patient's rash is due to irritation from substances like harsh soaps, detergents, or even the hot concrete she walked on barefoot.
  • Seborrheic Dermatitis: Less likely given the distribution and the fact that it typically involves areas rich in sebaceous glands, but could be considered if the rash has a different appearance in certain areas.

• Do Not Miss Diagnoses

  • Stevens-Johnson Syndrome (SJS) or Toxic Epidermal Necrolysis (TEN): Although rare and not as likely given the lack of systemic symptoms and mucosal involvement, these conditions are life-threatening and require immediate attention. The patient's symptoms of congestion and phlegm could be unrelated, but any sign of mucosal involvement or systemic illness would necessitate consideration of SJS/TEN.
  • Drug Reaction with Eosinophilia and Systemic Symptoms (DRESS): This is another severe condition that could present with a rash and systemic symptoms. Although the patient denies new medications, any recent medication use, even over-the-counter, should be thoroughly investigated.

• Rare Diagnoses

  • Pityriasis Rosea: Characterized by a herald patch followed by a secondary eruption of oval-shaped lesions, this condition could be considered if the rash has a distinctive appearance, although the intense pruritus and initial involvement of the feet are not typical.
  • Dyshidrotic Eczema (Pompholyx): This condition involves the palms and soles with small, itchy vesicles, but the patient's rash has spread beyond these areas, making this a less likely diagnosis.