Suppose that a mutant organism is unable to make the amino acid arginine. Knowing that the metabolic pathway to the production of arginine is ornithine citrulline argininosuccinate arginine, you test the ability of the mutant to grow in the presence of each one of these compounds, providing just one of the compounds in each of your tests. Also suppose that you find that the mutant can grow in the presence of argininosuccinate or arginine, but NOT in the presence of citrulline or ornithine. From this information, which step in the metabolic pathway do you think is blocked?