= Solution
\b[<Succinate> is the most plausible substrate.] Its oxidation by <succinate dehydrogenase>, or <respiratory complex II>, supplies electrons to the <ubiquinone> pool, bypassing <respiratory complex I>. Electron passage through <respiratory complex III> and <cytochrome c oxidase> supports a lower phosphorylation yield than entry through <NADH> at <respiratory complex I>, consistent with an apparent <phosphorylation-to-oxidation ratio> around $1.5$. The later involvement of a <mitochondrial dicarboxylate carrier> is additional evidence because <succinate> is one of its transported substrates. The ratio alone is not a chemically unique identification: other electron-entry routes and altered coupling can also produce a similar yield. Confirm by comparing defined substrates and sensitivity to a <succinate dehydrogenase> inhibitor.
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