Differential Diagnosis for a 22-year-old Female with Hives

The patient presents with hives in the morning and at night for the past 2 weeks, which have been worsening. Given her medication regimen and symptoms, the following differential diagnoses are considered:

• Single Most Likely Diagnosis
  • Allergic Reaction to Medication: The patient is on multiple medications, including venlafaxine, lamotrigine, oxybutynin ER, and hydroxychloroquine. Any of these could potentially cause an allergic reaction, manifesting as hives. The timing of the hives (morning and night) could correlate with the timing of medication intake.
• Other Likely Diagnoses
  • Urticaria: This is a common condition characterized by itchy hives, which can be acute or chronic. The cause can be allergic, but often no specific allergen is identified. The patient's symptoms of worsening hives over two weeks could fit into this category.
  • Atopic Dermatitis: Although typically presenting with eczematous lesions, atopic dermatitis can sometimes manifest with hives, especially if there's an allergic component or exacerbation.
  • Insect Bites or Stings: Repeated exposure to insect bites or stings could cause hives, especially if the patient is allergic to the venom.
• Do Not Miss Diagnoses
  • Anaphylaxis: Although the patient's symptoms are currently limited to hives, any sign of an allergic reaction should prompt consideration of anaphylaxis, a life-threatening condition that requires immediate intervention.
  • Stevens-Johnson Syndrome (SJS) or Toxic Epidermal Necrolysis (TEN): These are severe skin conditions usually triggered by medications. While the patient's current symptoms do not fully align with SJS/TEN, any new skin rash in the context of medication use should prompt consideration of these conditions due to their potential severity.
  • Autoimmune Disorders: Certain autoimmune conditions, like lupus (which hydroxychloroquine is sometimes used to treat), can cause skin rashes, including hives. However, these conditions typically present with a broader range of symptoms.
• Rare Diagnoses
  • Mastocytosis: A condition characterized by the accumulation of mast cells in one or more organs, which can cause hives among other symptoms. It's rare and usually presents with more systemic symptoms.
  • Erythema Multiforme: An immune-mediated disorder that can cause skin lesions, including hives, often in response to infections or medications. It's less common and typically has a distinct "target lesion" appearance.

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 her hives.