Final answer:
Hematuria is the urinalysis finding that would suggest trauma to the kidneys but not the urinary bladder, often associated with accidents causing damage to the renal system.
Step-by-step explanation:
The best urinalysis finding that would most likely suggest trauma to the kidneys from the patient's motorcycle accident, but not to the urinary bladder, is hematuria. This condition is characterized by the presence of blood in the urine and can be a result of kidney trauma, such as from an accident. Hematuria is specifically seen in cases that involve injury to the renal system, which might occur with rib and pelvic fractures, as the kidneys could experience damage from blunt trauma.
Conditions like pyuria (presence of pus in the urine) and albuminuria (presence of albumin in the urine) can indicate infections or kidney disease respectively, but are less specific to trauma. On the other hand, uremia (presence of an excess amount of urea in the blood) can arise from various conditions including kidney failure, dehydration, and obstruction of the urinary tract, like from kidney stones or tumors, and thus is also not specific to trauma. Phenylketonuria is a genetic disorder and is unrelated to trauma.