I'm sorry that your question wasn't answered in a timely manner. You've described a cutaneous drug reaction - a cutaneous or mucocutaneous reaction to a topical, oral, or injectable drug/vaccine. An adverse drug reaction can occur after one treatment, after several treatments, or after years of treatment. Clinical signs are extremely variable and may include papules, plaques, pustules, vesicles, bullae, purpura, erythema, urticaria, angiodedema, alopecia, erythema multiforme or toxic epidermal necrolysis lesions, scaling or exfoliation, erosions, ulceration, and otitis externa. It's unclear to me which of the above her "large red welts" represent but angioedema and alopecia, at least, are present. Her very low white blood cell count (leukopenia) is consistent with a drug reaction.
Treatment consists of discontinuing the use of all suspect drugs administered within 2-4 weeks before lesion development, symptomatic and supportive care (e.g., whirlpool baths, fluids, electrolytes, and parenteral nutrition) as well as preventing secondary bacterial skin infection with systemic antibiotics (the Clavamox). In more severe cases, treatment with prednisone at an immunosuppressive dose (1 mg/lb daily) may be helpful. Significant improvement may be seen within 1-2 weeks. After lesions resolve (~2-8 weeks) the dosage of prednisone should be gradually tapered over a period of 4-6 weeks. In most cases, steroid therapy can be discontinued. The prognosis is good except with multiorgan involvement or extensive epidermal necrosis.
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