About 3 Years Ago, a 39-Year-Old Construction Worker Became Increasingly $24 / 30$

About 3 years ago, a 39-year-old construction worker became increasingly uncoordinated. His wife describes bouts of depression and apathy beginning about a decade ago. Laboratory tests are normal. MRI and CT reveal striatal and caudate atrophy with "boxcar ventricles." His mini-mental status examination score is $24 / 30$ . The cranial nerve examination shows dysarthria, saccadic extraocular eye movements, and a hyperactive gag reflex. There is increased tone in all extremities. Polymerase chain reaction reveals one normal band with $20 \mathrm{CAG}$ (trinucleotide) repeats and the other with 49 CAG repeats. Modulation of respiration and mitochondrial membrane potential, and bioenergetic failure are associated with the abnormal gene in this disease. Which of the following mechanisms used to establish the mitochondrial electrochemical gradient may be altered in this disease?

A) The action of ATP synthase
B) Transfer of electrons from NADH to $\mathrm{O}_{2}$ in the intermembrane space
C) Pumping of protons into the mitochondrial matrix by respiratory chain activity
D) Proton-translocating activity in the inner membrane
E) Transport of ATP out of the matrix compartment by a specific transporter