A 15-year-old girl is brought to a pediatric clinic with severe abdominal pain,nausea,and vomiting.She does not have a fever,peritoneal signs,or an elevated white blood cell count.The symptoms resolve in 48 hours.She has a history of multiple transient episodes of facial edema without pruritus.Laboratory examination is most likely to reveal which of the following abnormalities in this patient?

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This patient's presentation and history are consistent with a diagnosis of hereditary angioneurotic edema,which is due to genetic or acquired deficiencies in C1 inhibitor (C1 INH).C1 INH normally blocks excessive C1 activation and proteolysis of C2 and C4.Reduced levels of C1 INH cause excessive proteolysis and reduced levels of C4 and C2,as well as generation of vasoactive peptides derived from C4 and C2.These peptides cause edema.Gastrointestinal symptoms are due to bowel wall edema.Laryngeal edema can be life threatening.C4 deficiency due to gene mutations,which constitutes the most common complement system deficiency,predisposes to systemic lupus erythematosus (SLE)or SLE-like illness.Deficiency of C3 is associated with severe,recurrent bacterial infections at an early age and immune complex glomerulonephritis.C3 nephritic factor is an autoantibody against the alternative pathway C3 convertase.Patients with this antibody present with an illness similar to C3 deficiency.Factor I deficiency is associated with a failure to regulate C3 convertase,so that C3 becomes depleted and patients suffer recurrent infections and glomerulonephritis.

A 15-Year-Old Girl Is Brought to a Pediatric Clinic with Severe