Final answer:
The source of the bleeding is most probably in the urethra.
Step-by-step explanation:
When a patient reports having blood in their urine only at the start of voiding that disappears until the next time they void, the nurse can infer that the source of the bleeding is most probably in the urethra. This is because the urethra is responsible for the transport of urine from the bladder to the outside of the body, and any bleeding in this area would likely be noticed at the start of voiding.