Final answer:
The lowest concentration of Na+ after a large meal would be in the ileum, the last part of the small intestine, due to active transport mechanisms that absorb sodium ions and other nutrients.
Step-by-step explanation:
After a large meal, you would expect the concentration of Na+ (sodium ions) to be the lowest in the ileum, which is the last part of the small intestine. This is due to the active transport mechanisms involved in absorbing nutrients and electrolytes throughout the small intestine.
As food passes through, sodium ions are accumulated inside the cells of the intestinal lining through co-transport mechanisms that typically result in a reduced concentration of sodium in the chyme. A sodium-potassium pump then works to pump sodium out of the cells, which is an ATP-dependent process. Following this active absorption stage, any remaining food waste along with some leftover sodium ions passes from the ileum into the colon, where much of the remaining water is absorbed.