Complex I is the NAD dehydrogenase complex of the Respiratory Chain.
The NADH.H+ that is oxidized in the electron transport chain should enter via Complex I:
Since the patient has a deficit in the activity of Complex I, NADH.H+ is not an appropriate source of reduction equivalents for the affected cells.
Enzymes (c) isocitrate dehydrogenase, (d) succinyl thiokinase and (g) malate dehydrogenase catalyze reactions in which NADH.H+ is produced. The NADH.H+ generated can not be used properly by the affected cells because of the decrease activity of Complex I.
Enzymes (a) Citrate Synthase, (b) Aconitase and (f) fumarase, do not catalyze reactions in which reduced cofactors are produced.
Enzyme (e) Succinate dehydrogenase catalyzes a reaction in which FADH2 is produced. In fact, succinate dehydrogenase is complex II in the respiratory chain. These reduction equivalents in FADH.H+ do not need to go through Complex I, since they go directly to CoQ, so the obtention of energy from them is not affected by the lack of activity in complex I.
This picture shows clearly how the production of reduced cofactors occurs in the Krebs Cycle: