A 60-year-old patient presents with a new onset of attacks of nonpruritic angioedema and a depressed C4 level. What is the first diagnosis you consider?

Acquired C1 esterase inhibitor deficiency. This situation is usually encountered in patients with lymphoma who have a circulating low-molecular-weight IgM, decreased C1 esterase inhibitor level, and low levels of the C1–C4. The mechanism of C1 activation is by reaction with immune complexes or by binding of the C1 to antiidiotypic antibody bound to the immunoglobulin on the surface of the tumor cells. Acquired C1 esterase deficiency has also been reported with connective tissue disorders such as systemic lupus erythematosus, with carcinoma, and with an IgG antibody directed toward C1 esterase inhibitor. In the latter circumstance, the C1 levels are usually normal. Androgen therapy benefits patients with acquired C1 esterase inhibitor deficiency by increasing C1 esterase inhibitor production by the liver.