Final answer:
The most immediate concern for the nurse should be the patient's elevated temperature, which indicates a potential infection, especially concerning due to the patient's history of splenectomy.
Step-by-step explanation:
The most immediate concern for the nurse assessing a 22-year-old patient who had a splenectomy is the oral temperature of 100°F (37.8°C). This is the most significant finding because the patient is at increased risk for infections due to the absence of the spleen, which plays a significant role in filtering bacteria from the blood and aiding in the immune response. The other findings, such as unprotected sex, elevated blood pressure, and daily marijuana use, present long-term health risks, but these are not as immediately concerning as the potential for infection indicated by the fever.