Final answer:
The prefix 'em' in empyema means 'in' or 'within', relating to the accumulation of pus within a body cavity. It is not directly related to the listed medical terms edema, embolus, episodic memory, or expressive aphasia.
Step-by-step explanation:
The prefix 'em' in the word empyema essentially means 'in' or 'within'. Empyema refers to a condition in which there is an accumulation of pus within a body cavity, usually in the pleural space, which is the area between the lungs and the chest wall. This term does not directly relate to the other medical terms provided like edema, embolus, episodic memory, or expressive aphasia, which describe fluid accumulation in tissue, a blockage in a blood vessel, memory of specific events, and a type of language impairment, respectively.