Identify the amino acid that is the major contributor to the transport of nitrogen destined for excretion as urea:

Nitrogen metabolism involves the safe transport of potentially toxic ammonia from peripheral tissues to the liver for neutralization. Ammonia is converted into organic forms to prevent neurotoxicity during transport in the blood. The urea cycle in the liver then converts this nitrogen into urea for renal excretion. Specific amino acids serve as specialized "carriers" to move nitrogen across the body's metabolic compartments. Rationale: A. Alanine is a significant nitrogen carrier, specifically from skeletal muscle to the liver via the glucose-alanine cycle. While important, it primarily handles nitrogen generated during muscle protein catabolism. It is not the "major" contributor to total systemic nitrogen transport compared to the more versatile and high-capacity glutamine pathway. B. Glycine is the simplest amino acid and participates in many biosynthetic pathways, including heme and creatine synthesis. While it contains nitrogen, it does not function as a dedicated carrier for the transport of excess ammonia for excretion. Its role in the urea cycle is indirect and not related to systemic transport. C. Lysine is an essential amino acid and is purely ketogenic. It does not participate in the transamination reactions that are central to nitrogen transport and urea cycle priming. Therefore, it cannot serve as a vehicle for moving excess nitrogen from peripheral tissues to the hepatocyte for processing into urea. D. Glutamine is the major contributor to systemic nitrogen transport. It carries two ammonia groups in a non-toxic form (as the amide of glutamate). Most tissues convert free ammonia into glutamine via glutamine synthetase for safe passage to the liver and kidneys, making it the most abundant amino acid in human plasma. E. Ornithine is a key intermediate within the urea cycle itself. It combines with carbamoyl phosphate to form citrulline inside the mitochondria. While essential for the production of urea, it acts as a catalytic carrier within the hepatocyte rather than a vehicle for transporting nitrogen through the systemic circulation.