A 25-year-old woman presents with a 2-week history of febrile illness and chest pain. She has an erythematous, macular facial rash and tender joints, paicularly in her left wrist and elbow. A CBC shows mild anemia and thrombocytopenia. Coicosteroids are prescribed for the patient. This medication induces the synthesis of an inhibitor of which of the following enzymes in inflammatory cells?