A 39-year-old woman sees her physician because of acute onset of severe dyspnea. On physical examination, she is afebrile and has marked laryngeal stridor and severe airway obstruction. The medical history indicates that she has had similar episodes since childhood and episodes of colicky gastrointestinal pain. Her mother and her brother are similarly affected. There is no history of severe or recurrent infections. She does not have urticaria. Laboratory studies show normal WBC count, hematocrit, and platelet count. A deficiency in which of the following plasma components is most likely to produce these findings?

Hereditary angioedema is a rare autosomal-recessive disorder of the complement pathway in which there is a deficiency of antigenic or functional C1 inhibitor, resulting in recurrent episodes of edema. Of the remaining choices, only C3 and IgA have a deficiency state. C3 deficiency is accompanied by recurrent infections with pyogenic bacteria. IgA deficiency leads to mild recurrent gastrointestinal and respiratory tract infections and predisposes to anaphylactic transfusion reaction. b2 -Microglobulin is a component of MHC class I; it can be increased with HIV infection and can be a substrate for amyloid fibrils in patients receiving long-term hemodialysis. 5-Hydroxytryptamine (serotonin) has an effect similar to histamine, which drives vasodilation and edema. IgE participates in localized or systemic anaphylaxis with edema.