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USMLE® Step 1 style questions USMLE

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A 7-month-old boy is brought to the office because of a recurrent pruritic rash on his body. His parent first noticed a few lesions 4 days ago that disappeared within 2 hours. On subsequent days, similar but widespread lesions occurred that improved within a few hours without any intervention. His parent states he has not had any fevers, fatigue, or loss of appetite recently. He recently started gradually weaning off breast milk exclusively and eats boiled eggs for breakfast. He was born via spontaneous vaginal delivery, and his parent reports he has been healthy with no medical conditions. Vitals are within normal limits. Physical examination shows well-demarcated, raised, erythematous plaques as shown:

Reproduced from: Wikimedia Commons

Which of the following is the most likely pathophysiology of this patient’s condition?

External References

First Aid









urticaria p. 489

Urticaria p. 487, 489

ethosuxamide p. 565

scombroid poisoning p. 248

serum sickness p. 111

sulfa drug allergies p. 253

as type I hypersensitivity p. 111


urticaria p. 487


Urticaria, commonly referred to as hives, is a kind of skin rash notable for pale red, raised, itchy bumps, which can cause a burning or stinging sensation. Lesions are frequently caused by allergic reactions; however, there are many nonallergic causes, such as autoimmune disorders like thyroid disease.

Most cases of hives lasting less than six weeks (acute urticaria) are the result of an allergic trigger. Chronic urticaria (hives lasting longer than six weeks) is rarely due to an allergy. Treatment for urticaria typically involves identifying and avoiding triggers, as well as taking medication to relieve symptoms. Antihistamines are often prescribed to block the release of histamine and reduce itching and swelling. In severe cases, oral or injectable corticosteroids may be prescribed.


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