Final answer:
The waste nitrogen from skeletal muscle amino acid catabolism is converted into urea in the liver through the urea cycle, which is then excreted by the kidneys.
Step-by-step explanation:
The waste nitrogen from skeletal muscle amino acid catabolism is diverted to the liver primarily in the form of ammonia, which is then rapidly converted into urea. The urea cycle is the central pathway in mammals to process nitrogen from the breakdown of proteins and facilitate its excretion from the body. The liver synthesizes urea from two ammonia molecules and a carbon dioxide molecule, releasing it into the bloodstream for eventual excretion by the kidneys in urine.