Final answer:
The surgeon must enter through the renal hilum.
Step-by-step explanation:
When removing a portion of the kidney, the surgeon must enter through the renal hilum. The renal hilum is the entry and exit site for structures servicing the kidneys, including vessels, nerves, lymphatics, and ureters. It is the recessed medial area of the kidney through which the renal artery, renal vein, ureters, lymphatics, and nerves pass.