Final answer:
Isocitrate lyase is the enzyme that characterizes organisms with the glyoxylate pathway, enabling them to convert acetyl-CoA to substrates for glucose synthesis. The correct answer is option d.
Step-by-step explanation:
The enzyme that differentiates organisms capable of carrying out the glyoxylate pathway from those that cannot is isocitrate lyase. This enzyme is crucial to the pathway as it bypasses the CO2-releasing steps of the citric acid cycle which occurs in mitochondria. Organisms carrying out the glyoxylate cycle can thus convert acetyl-CoA to precursor substrates for gluconeogenesis. The glyoxylate pathway allows organisms, especially plants, bacteria, and fungi, to convert fats into carbohydrates, which is particularly useful when sugars are scarce.
Other enzymes mentioned, such as glyoxylate synthase (which does not exist), glyceraldehyde-3-phosphate dehydrogenase, and citrate synthase, serve different functions. Glyceraldehyde-3-phosphate dehydrogenase is involved in glycolysis and citrate synthase initiates the citric acid cycle, both of which are critical pathways of cellular respiration in all cells.
Therefore, the correct option reflecting the enzyme that characterizes the glyoxylate pathway is (d) Isocitrate lyase.