Final answer:
The renal pelvis, located in the hilum of the kidney, would be the first region impacted by a blockage in the ureter due to a tumor.
Step-by-step explanation:
If a ureter was blocked due to the development of a tumor, the region of the kidney that would be impacted first is the renal pelvis. This area is where the urine collects after being filtered from the blood and before flowing into the ureter. The renal pelvis is located in the hilum region, which is where blood vessels, nerves, and ureters bunch before entering or exiting the kidney.
The hilum region acts as a channel for urine to be excreted from the kidney into the ureter. If the renal pelvis is blocked, urine would back up into the kidney which could lead to increased pressure and potential damage to the kidney structures, including the calyces and eventually the nephrons. Kidney stones, for example, can form in the kidney and grow large enough to block the renal pelvis or ureter, illustrating how such obstructions can occur.