Final answer:
After a myelography using a water-soluble contrast dye, the nurse should place the client in a lying flat position with the head slightly elevated.
Step-by-step explanation:
After a myelography using a water-soluble contrast dye, the nurse should place the client in a lying flat position with the head slightly elevated. This position helps to prevent the contrast dye from settling in the client's lower back, which could impede the flow of cerebrospinal fluid. By keeping the client in a flat position, the contrast dye can disperse evenly and the client's back pain can be relieved.