To solve this problem, we need to identify the step in the tricarboxylic acid (TCA) cycle that does not involve oxidation of the substrate. Let's analyze each option:$\newline \bullet$Option 1: Succinyl-CoA$\rightarrow$Succinic acid$\newline$ - This step involves the conversion of Succinyl-CoA to Succinic acid.$\newline$ - It is coupled with the substrate-level phosphorylation of GDP to GTP (or ADP to ATP).$\newline$ - Importantly, this step does not involve the oxidation of the substrate.$\newline \bullet$Option 2: Isocitrate$\rightarrow \alpha$-ketoglutaric acid$\newline$ - This step involves the oxidation of Isocitrate to \alpha-ketoglutaric acid.$\newline$ - NAD$^+$ is reduced to NADH in this process.$\newline$ - Therefore, this step involves oxidation.$\newline \bullet$Option 3: Malic acid$\rightarrow$Oxaloacetic acid$\newline$ - This step involves the oxidation of Malic acid to Oxaloacetic acid.$\newline$ - NAD$^+$ is reduced to NADH in this process.$\newline$ - Therefore, this step involves oxidation.$\newline \bullet$Option 4: Succinic acid$\rightarrow$Malic acid$\newline$ - This step involves the conversion of Succinic acid to Fumaric acid, followed by hydration to Malic acid.$\newline$ - FAD is reduced to FADH$_2$ during the conversion of Succinic acid to Fumaric acid.$\newline$ - Therefore, this step involves oxidation.$\newline$Based on the analysis, the step Succinyl-CoA$\rightarrow$Succinic acid does not involve oxidation of the substrate.$\newline$Thus, the correct answer is Option 1.