This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Biochemistry
Cytochrome oxidase contains the following metal ion for transfer of four electrons from cytochrome c to O2: 
Select and explain. 
A. Fe, B. Cu, C. Mg, D. Mn
B: Cu
Answer: B: Cu 
Explanation: Reduced cytochrome c is oxidized by Complex IV (cytochrome c oxidase), with the concomitant reduction of O2 to two molecules of water. This transfer of four electrons from cytochrome c to O2 involves two heme groups, a and a3, and Cu. Electrons are passed initially to a Cu center (CuA), which contains 2Cu atoms linked to two protein cysteine-SH groups (resembling an Fe-S), then in sequence to heme a, heme a3, a second Cu center, CuB, which is linked to heme a3, and finally to O2. Ref: Botham K.M., Mayes P.A. (2011). Chapter 13. The Respiratory Chain & Oxidative Phosphorylation. In D.A. Bender, K.M. Botham, P.A. Weil, P.J. Kennelly, R.K. Murray, V.W. Rodwell (Eds), Harper's Illustrated Biochemistry, 29e.