Differential Diagnosis for Wheals Localized to Hip

The patient presents with wheals (hives) localized to the hip, which are present in the morning but fade by afternoon. Considering this specific presentation, the differential diagnosis can be organized into the following categories:

• Single Most Likely Diagnosis

  • Delayed Pressure Urticaria (DPU): This condition is characterized by the development of wheals at sites of sustained pressure, typically appearing 4-6 hours after the pressure has been applied and resolving within 24-48 hours. The fact that the wheals are localized to the hip and appear in the morning (after a night of pressure from sleeping) but fade by afternoon fits the typical presentation of DPU.

• Other Likely Diagnoses

  • Dermatographism: Although less likely given the specific localization and timing, dermatographism (skin writing) could be considered if the patient's symptoms are exacerbated by scratching or rubbing, which might occur more frequently in the morning.
  • Insect Bites or Stings: Repeated bites or stings in the same area could cause localized urticaria, but the pattern of being present in the morning and fading by afternoon is less typical for this diagnosis.

• Do Not Miss Diagnoses

  • Angioedema: While angioedema typically presents with deeper tissue involvement and can be life-threatening if it involves the airway, any form of urticaria that could potentially progress to angioedema should be considered, especially if there are systemic symptoms or a history of similar episodes.
  • Urticarial Vasculitis: This is an inflammation of the blood vessels and can present similarly to urticaria but with a more prolonged duration of individual lesions and potential systemic symptoms. It's crucial to differentiate this from simple urticaria due to its implications for treatment and prognosis.

• Rare Diagnoses

  • Erythema Multiforme: Although typically presenting with target lesions, erythema multiforme could be considered in the differential for any skin eruption, especially if there are mucosal involvement or systemic symptoms.
  • Mastocytosis: This condition, characterized by an accumulation of mast cells in the skin, can cause urticaria-like lesions, but it's much rarer and usually presents with other systemic or cutaneous manifestations.

Each of these diagnoses should be considered in the context of the patient's full medical history, physical examination, and potentially, further diagnostic testing to determine the underlying cause of the symptoms.