## Core Concept
The Kreb's Citric acid cycle (also known as the Tricarboxylic acid cycle or TCA cycle) is a key metabolic pathway that generates energy through the oxidation of acetate derived from carbohydrates, fats, and proteins into carbon dioxide and water. The cycle takes place in the mitochondria and is a crucial part of cellular respiration. One of the steps in this cycle is physiologically irreversible.

## Why the Correct Answer is Right
The only physiologically irreversible reaction in the Kreb's Citric acid cycle is the conversion of **α-ketoglutarate** to **succinyl-CoA**, catalyzed by the enzyme **α-ketoglutarate dehydrogenase**. This reaction is irreversible under physiological conditions due to the large negative change in Gibbs free energy. α-Ketoglutarate dehydrogenase is a multi-enzyme complex that requires NAD+ as a coenzyme, converting it to NADH, and also involves the decarboxylation of α-ketoglutarate, producing CO2 and succinyl-CoA.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because it does not correspond to the substrate for the irreversible step in the Kreb's cycle.
- **Option B:** This option is incorrect because it represents another intermediate in the cycle but not the substrate for the irreversible reaction.
- **Option D:** This option is incorrect because it is not directly related to the substrate for the physiologically irreversible reaction in the Kreb's cycle.

## Clinical Pearl / High-Yield Fact
A key point to remember is that **α-ketoglutarate dehydrogenase** is not only crucial for the Kreb's cycle but also is a regulatory step and a point of convergence for several metabolic pathways. Inhibition or deficiency of this enzyme can lead to significant metabolic disturbances.

## Correct Answer Line
**Correct Answer: C. α-ketoglutarate**.