**Core Concept**
NADH-Q oxidoreductase (Complex I) is the first enzyme in the electron transport chain that transfers electrons from NADH to ubiquinone (Q), and it is responsible for pumping protons across the inner mitochondrial membrane. This step is critical for establishing the proton gradient used to drive ATP synthesis.

**Why the Correct Answer is Right**
NADH-Q oxidoreductase (Complex I) transfers four protons from the mitochondrial matrix to the intermembrane space per molecule of NADH oxidized. It catalyzes the transfer of electrons from NADH to ubiquinone (Q), simultaneously pumping 4 H⁺ ions across the inner mitochondrial membrane via its membrane-spanning domains. This proton translocation is a key component of oxidative phosphorylation and contributes significantly to the proton gradient.

**Why Each Wrong Option is Incorrect**
Option B: Cytochrome c oxidase (Complex IV) transfers electrons from cytochrome c to oxygen but only contributes to 2 H⁺ translocation, not 4.
Option C: "Cytochrome C - Q oxidoredictase" is a miswritten version of cytochrome c oxidase; it is not a valid enzyme and does not transfer four protons.
Option D: Isocitrate dehydrogenase is a glycolytic enzyme in the Krebs cycle that produces NADH, but it does not transfer protons directly and is not involved in electron transport.

**Clinical Pearl / High-Yield Fact**
Remember: Complex I (NADH-Q oxidoreductase) pumps **4 protons** per NADH, while Complex III pumps 4 protons per 2 electrons (2 H⁺ per electron), and Complex IV contributes only 2 protons. This makes Complex I the largest proton pump in the chain.

✓ Correct Answer: A. NADH-Q Oxidoreductase