Final answer:
High NADH levels allosterically inhibit several enzymes in the citric acid cycle, but not malate dehydrogenase; hence, the correct answer is D) malate dehydrogenase.
Step-by-step explanation:
The question is asking which of a set of enzymes involved in the regulation of the citric acid cycle and pyruvate oxidative decarboxylation is not allosterically inhibited by a high concentration of NADH. High NADH concentration regulates pyruvate oxidative decarboxylation and the citric acid cycle by allosterically inhibiting pyruvate dehydrogenase, isocitrate dehydrogenase, and α-ketoglutarate dehydrogenase. However, malate dehydrogenase is not allosterically inhibited by high NADH levels. Instead, it acts later in the Krebs cycle, converting malate back into oxaloacetate and generating additional NADH. Therefore, the enzyme among the options given that is not allosterically inhibited by high NADH concentration is D) malate dehydrogenase.