Differential Diagnosis for the Patient's Condition

The patient presents with an increasing red spot on her left hip/lower torso, which she initially attributed to a friction burn due to biking and wearing a seatbelt. However, the area has spread, increased in erythema, is warm to touch, and pruritic but not painful. Based on these symptoms, the following differential diagnoses are considered:

• Single Most Likely Diagnosis

  • Contact dermatitis: This is the most likely diagnosis given the patient's history of recent biking and seatbelt use, which could cause friction and irritate the skin, leading to an allergic or irritant contact dermatitis. The pruritic nature of the rash and its progression also support this diagnosis.

• Other Likely Diagnoses

  • Cellulitis: Although the area is not painful to touch, the warmth and erythema could suggest a mild cellulitis, especially if there was a break in the skin that became infected.
  • Eczema (atopic dermatitis): The pruritic nature of the lesion could also suggest eczema, particularly if the patient has a history of atopic diseases.
  • Intertrigo: Given the location on the hip/lower torso, intertrigo (an inflammatory rash in the body folds) could be considered, especially if the area is prone to moisture.

• Do Not Miss Diagnoses

  • Necrotizing fasciitis: Although rare and less likely given the absence of severe pain, necrotizing fasciitis is a life-threatening condition that must be considered with any rapidly spreading erythematous lesion, especially if there's a history of trauma (like friction from biking).
  • Deep vein thrombosis (DVT): The warmth and erythema could also be signs of a DVT, particularly if the patient has been immobile for periods (e.g., long bike rides) or has other risk factors for DVT.

• Rare Diagnoses

  • Erythema migrans (Lyme disease): If the patient has been outdoors in areas where ticks are common, erythema migrans could be considered, although it typically presents as a distinctive "bull's-eye" rash.
  • Fixed drug eruption: If the patient has recently started any new medications, a fixed drug eruption could be a rare cause of the lesion, characterized by its tendency to recur in the same location with drug re-exposure.