A patient presents with history of episodic painful edema of face and larynx. Which of the following is likely to be deficient in this patient?

History of recurrent episodes of painful edema of face and larynx suggests a diagnosis of hereditary angioneurotic edema which is caused by deficiency of C1 esterase inhibitor. C1 esterase inhibitor (C1 INH) is an alpha-globulin, which controls the first stage of the classic complement pathway and inhibits thrombin, plasmin, and kallikrein. Deficiency results in spontaneous activation of C1, leading to consumption of C2 and C4. It is decreased in Hereditary angioedema. Ref: Pocket Guide to Diagnostic Tests, 5th Edition By Diana Nicoll, 5th Edition