Final answer:
The nurse should rank 'Risk for Infection' as the nursing diagnosis with the highest priority for a client who has recently undergone a mastectomy.
Step-by-step explanation:
The nurse should rank 'Risk for Infection' as the nursing diagnosis with the highest priority for a client who has recently undergone a mastectomy. This is because after surgery, the client is at risk for developing an infection at the surgical site. Infection can delay the healing process and lead to further complications. The nurse should closely monitor the client for signs of infection such as redness, warmth, and increased pain at the surgical site.