Differential Diagnosis for Patient with Hives

The patient had poison ivy on their right arm two weeks ago, which resolved, and now presents with flesh-colored hives all over their body, accompanied by itching. Here's a differential diagnosis based on the provided information:

• Single Most Likely Diagnosis

  • Idiopathic Urticaria: This is the most likely diagnosis given the sudden onset of itchy, flesh-colored hives without an obvious cause. The fact that the patient had poison ivy two weeks prior, which has since resolved, suggests that the current symptoms might not be directly related to the poison ivy, making idiopathic urticaria a plausible explanation.

• Other Likely Diagnoses

  • Allergic Reaction to Another Substance: Given the patient's recent history with poison ivy, it's possible they've been exposed to another allergen, leading to an allergic reaction manifesting as hives.
  • Viral Exanthem: Some viral infections can cause rashes that resemble hives. The timing, two weeks after the poison ivy resolved, could suggest a viral etiology, especially if the patient has been exposed to someone with a viral illness.
  • Autoimmune Urticaria: This condition occurs when the body's immune system attacks its own tissues, leading to hives. The recent poison ivy could potentially trigger an autoimmune response, although this is less common.

• Do Not Miss Diagnoses

  • Anaphylaxis: Although the patient's symptoms are currently limited to itchy hives, anaphylaxis is a life-threatening condition that can present with urticaria among other symptoms like swelling, difficulty breathing, and a drop in blood pressure. It's crucial to monitor for signs of anaphylaxis, especially if the patient has a history of severe allergic reactions.
  • Mastocytosis: A condition characterized by an accumulation of mast cells in one or more organs, which can lead to episodes of urticaria, among other symptoms. While rare, it's a diagnosis that should not be missed due to its potential for severe complications.

• Rare Diagnoses

  • Urticarial Vasculitis: An inflammation of the blood vessels that can cause hives, but these are typically more painful than itchy and can leave behind bruising. This condition is rare but should be considered if the hives are atypical or if there are systemic symptoms.
  • Erythema Multiforme: A skin condition characterized by target lesions, which can sometimes resemble hives. It's often associated with infections or medications and can be severe, making it a rare but important diagnosis to consider.