Final answer:
The nurse should document the odor of the urine from an infant with untreated phenylketonuria (PKU) as mousy or musty, which is a characteristic sign of this condition.
Step-by-step explanation:
A 3-week-old infant admitted with untreated phenylketonuria (PKU) will typically have urine that has a characteristic mousy or musty odor.
This distinct odor is a result of the accumulation of phenylalanine in the body, which occurs due to the lack of the enzyme needed to break down this amino acid in individuals with PKU.
The nurse should document the urine odor as mousy or musty, which is a clinical sign of untreated PKKU.