Final answer:
A biopsy of urticarial skin lesions is warranted when the diagnosis is unclear, when chronic symptoms do not respond to treatment, or to differentiate between allergic and non-allergic reactions. It is particularly crucial in atypical cases or when there's a suspicion of underlying systemic diseases.
Step-by-step explanation:
Biopsy (bx) of urticarial skin lesions should be performed when a diagnosis cannot be made based on history and physical exam alone, or when chronic urticaria persists despite routine allergy testing. It can be particularly important if symptoms are atypical, do not respond to standard treatments, or if there is suspicion of urticarial vasculitis, an underlying systemic disease, or to distinguish between allergic and non-allergic causes.
In cases such as the one where a patient exhibits an itchy rash that does not improve with treatment, a biopsy might assist in uncovering the underlying condition. Similarly, if skin testing, which involves injecting allergen extracts into the epidermis to elicit a wheal and flare response, yields inconclusive results, or there's a concern for conditions like rabies following a dog bite, skin biopsy for further examination becomes crucial. Additionally, when a patient experiences persistent rashes that are possibly photo-aggravated, as seen in certain autoimmune conditions, a biopsy may be indicated to reach an accurate diagnosis.