**Question:** All of the following are enzymes of TCA cycle, EXCEPT

A. Citrate synthase (CS)
B. Isocitrate dehydrogenase (ICDH)
C. Oxaloacetate decarboxylase (ODC)
D. Pyruvate dehydrogenase (PDH)

**Core Concept:** The TCA cycle (also known as the Krebs cycle or citric acid cycle) is a crucial process in cellular respiration, where energy-rich molecules like glucose are converted into ATP, NADH, and FADH2. It involves a series of enzyme-catalyzed reactions that take place in the mitochondria.

**Why the Correct Answer is Right:** The correct answer, ODC (Oxaloacetate decarboxylase), is not part of the TCA cycle enzymes because it is involved in the citrulline synthesis pathway, not the TCA cycle. In contrast, CS (Citrate synthase), ICDH (Isocitrate dehydrogenase), and PDH (Pyruvate dehydrogenase) are crucial enzymes in the TCA cycle.

**Why Each Wrong Option is Incorrect:**

A. Citrate synthase (CS) is a key enzyme in the TCA cycle, catalyzing the first step and forming citrate from oxaloacetate and acetyl-CoA.

B. Isocitrate dehydrogenase (ICDH) is responsible for the third step of the TCA cycle, converting isocitrate into α-ketoglutarate with the reduction of NAD+ to NADH.

C. Pyruvate dehydrogenase (PDH) is an essential enzyme in the TCA cycle, converting pyruvate into α-ketoglutarate, with the conversion of NAD+ to NADH and the decarboxylation of pyruvate.

D. Pyruvate dehydrogenase (PDH) is an essential enzyme in the TCA cycle, converting pyruvate into α-ketoglutarate, with the conversion of NAD+ to NADH and the decarboxylation of pyruvate.

**Core Concept:**

The TCA cycle (Krebs cycle or citric acid cycle) is a series of enzyme-catalyzed reactions that occur in the mitochondria, generating ATP, NADH, and FADH2 to produce energy for cellular respiration.

**Why the Correct Answer is Right:** ODC (Oxaloacetate decarboxylase) is not part of the TCA cycle, as it is involved in citrulline synthesis, not the TCA cycle.

**Why Each Wrong Option is Incorrect:**

A. Citrate synthase (CS) is the initial enzyme in the TCA cycle, catalyzing the condensation of oxaloacetate and acetyl-CoA to form citrate, which is then converted to isocitrate by isocitrate dehydrogenase (ICDH).

B. Pyruvate dehydrogenase (PDH) is the key enzyme in the TCA cycle, converting pyruvate to α-ketoglutarate