**Question:** Electron transport from cytochrome b to cytochrome c is inhibited by

A. Cyanide
B. Azide
C. Oxygen
D. Proximal drugs

**Core Concept:** Electron transport chain is a series of protein complexes in the inner mitochondrial membrane that facilitates the movement of electrons, protons, and oxygen molecules. This process generates ATP, the cell's primary energy currency. Cytochrome b and c are components of the electron transport chain, with cytochrome b receiving electrons from complex I and cytochrome c acting as an electron carrier to complex III.

**Why the Correct Answer is Right:** Cyanide (A) inhibits electron transport chain by binding to cytochrome c, preventing its interaction with cytochrome b and complex III. This results in a decrease in oxygen consumption and an increase in oxygen levels, which is the basis for its use in clinical practice as a diagnostic tool in measuring tissue hypoxia.

**Why Each Wrong Option is Incorrect:**

B. Azide (N3) also inhibits electron transport chain by binding to cytochrome c, preventing its interaction with cytochrome b and complex III. This leads to a similar effect as seen with cyanide, including reduced oxygen consumption and increased oxygen levels.

C. Oxygen (O2) is an essential reactant in the electron transport chain, specifically in complex IV (cytochrome c oxidase). Inhibiting oxygen would be counterproductive to the functioning of the electron transport chain.

D. Proximal drugs (e.g., methotrexate, amphotericin B, and allopurinol) inhibit the activity of specific enzymes in the electron transport chain, such as dihydroorotate dehydrogenase, sterol biosynthesis pathway, and xanthine oxidase, respectively. These drugs do not directly inhibit cytochrome c or the interaction between cytochrome c and complex III.

**Clinical Pearl:** The correct answer, cyanide (A), helps clinicians in measuring tissue hypoxia, while azide (B) and cyanide (A) are often used as chemical inhibitors to study the electron transport chain in cellular and molecular biology research. Proximal drugs (D) target specific enzymes involved in cellular pathways, not cytochrome c or complex III.