Krebs cycle knowledge check — Which compound listed below is NOT an intermediate of the citric acid (Krebs) cycle?

Introduction / Context:
The citric acid cycle comprises a defined set of intermediates that interconvert as acetyl-CoA is oxidized. Recognizing bona fide intermediates helps avoid confusing TCA compounds with unrelated metabolites such as fatty acids.

Given Data / Assumptions:

• Canonical TCA intermediates include citrate, isocitrate, alpha-ketoglutarate, succinyl-CoA, succinate, fumarate, malate, and oxaloacetate.
• Stearate is an 18-carbon saturated fatty acid used in lipid metabolism, not the TCA cycle.

Concept / Approach:
Compare each option to the established TCA list. Any fatty acid (e.g., stearate) undergoes beta-oxidation to acetyl-CoA but is not itself a TCA intermediate.

Step-by-Step Solution:

Verification / Alternative check:
Standard metabolic maps place stearate in the lipid metabolism pathway, not in the TCA cycle diagram.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing substrates that feed into TCA (like acetyl-CoA derived from fatty acids) with the intermediates inside the cycle.

Final Answer:
Stearate.