Final answer:
In the context of a 5-month-old boy with an undescended left testis, true cryptorchidism is indicated by the inability to milk the testis down the inguinal canal.
Step-by-step explanation:
The nurse caring for a 5-month-old boy with an undescended left testis would identify true cryptorchidism as a situation in which the testis cannot be "milked" down the inguinal canal. This means that the testis is not palpable in the inguinal canal and cannot be brought down into the scrotum, indicating that it is undescended. True cryptorchidism contrasts with conditions where the testis can be brought into the scrotum or where other conditions like a venous varicosity or fluids are detected, which do not directly imply an undescended testis.