Final answer:
A needle must be attached to the rubber sleeve of a syringe for medical applications to allow for precise delivery or extraction of fluids.
Step-by-step explanation:
To answer the question, you need to attach a needle to the rubber sleeve. In medical applications, a rubber sleeve is often part of a syringe. The needle is the component that pierces the skin and allows for the delivery or extraction of fluids. When using a syringe, the needle is carefully screwed or clicked onto the end of the syringe, which has a rubber sleeve or a similar mechanism to ensure a tight and secure fit. It's essential for safety and accuracy in medical procedures that the needle is properly attached to the syringe before use.