This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Mechanisms of Enzyme Action, Subject name: Biochemistry
Nitric oxide synthase: 
Select and explain. 
A. Is inhibited by Ca++, B. Catalyze a de oxygenase reaction, C. Accept electron from NADH., D. Requires NADPH FAD, FMN, Heme iron.
D: Requires NADPH FAD, FMN, Heme iron.
Answer: D: Requires NADPH FAD, FMN, Heme iron. 
Explanation: Ans. D. Requires NADPH FAD, FMN, Heme ironNitric oxide synthase is a very complex cytosolic enzyme which requires five redox co-factors: NADPH, FAD, FMN, Heme and tetrahydrobiopterin (FH4 ). NOS activity depends on increased conc, of Ca2++ ions.