**Question:** Electron transport chain involves all except:

A. Ubiquinone
B. Cytochrome c
C. ATP synthase
D. NADH

**Core Concept:**
The electron transport chain (ETC) is a crucial component of cellular respiration, where energy is extracted from organic molecules to produce ATP (Adenosine Triphosphate), the primary energy currency of cells. The ETC is composed of several proteins and coenzymes, including Ubiquinone, Cytochrome c, and ATP synthase. NADH is also an essential electron carrier in the ETC.

**Why the Correct Answer is Right:**
Option D (NADH) is incorrect because NADH (Nicotinamide Adenine Dinucleotide) is a coenzyme involved in the Krebs cycle and electron transport chain. It plays a crucial role in transporting electrons from the cytoplasm to the mitochondria, ultimately contributing to ATP production.

**Why Each Wrong Option is Incorrect:**
A. Ubiquinone (Coenzyme Q) helps facilitate the transport of electrons between complex I and II, acting as an electron carrier.
B. Cytochrome c is a protein that carries electrons between complex III and IV, facilitating electron transport.
C. ATP synthase (F0F1-ATP synthase) is a complex enzyme responsible for synthesizing ATP from ADP and inorganic phosphate using the proton gradient generated during oxidative phosphorylation.

**Clinical Pearl:**
Understanding the role of each component in the electron transport chain is essential for grasping cellular respiration and its implications in clinical medicine. For example, deficiencies in these proteins can lead to various diseases and disorders, such as Leigh disease due to mitochondrial DNA mutations affecting complex II, III, or IV, or Leber hereditary optic neuropathy, caused by mutations in the MT-ND4 gene encoding complex I.