Final answer:
After a patient receives combination chemotherapy before a HSCT, the nurse's priority is to prevent infection due to the compromised immune system.
Step-by-step explanation:
The priority for a nurse after a patient receives combination chemotherapy before a hematopoietic stem cell transplant (HSCT) is A. Prevent patient infection. Post-chemotherapy, the patient has a compromised immune system due to the destruction of their bone marrow, increasing susceptibility to infections. Preventing infection is crucial since it's during this period that the patient's immune defenses are at their weakest. Measures such as maintaining a sterile environment, using prophylactic antibiotics, and limiting exposure to potential infection sources are key nursing interventions. Additionally, while avoiding abnormal bleeding and providing companionship are important, they are secondary to the pressing need to prevent life-threatening infections.