Final answer:
For a patient with subarachnoid hemorrhage who develops a fever, the primary concern is the resultant increase in cerebral oxygen consumption, which could worsen brain injury by demanding more oxygen when supply might already be compromised.
Step-by-step explanation:
For a patient admitted with subarachnoid hemorrhage and developing a fever of 38.5° C, the greatest concern among the provided options would be the resultant increase in cerebral oxygen consumption.
This is because an increase in body temperature can significantly raise the body's metabolic rate, which in turn elevates the cerebral metabolic rate for oxygen. In the context of a subarachnoid hemorrhage, the brain is already at risk for additional damage, and an increased metabolic rate could exacerbate the injury by demanding more oxygen at a time when the brain may already be struggling to receive adequate supply due to impaired blood flow or increased intracranial pressure. Therefore, the risk of additional brain injury due to increased oxygen consumption makes this the most serious concern.
Additionally, a fever can reset the body's thermostat via the hypothalamus, which may be beneficial in fighting infection, and fever-induced changes to immune system activity are also crucial. However, these changes are not as immediately concerning as the potential for increased cerebral damage in a patient with subarachnoid hemorrhage.