In scientific article/manuscript writing, the introduction plays a crucial role. It is strategically placed to do more than just introduce your manuscript. It is expected to fulfil the following:
Example:
You observe that A, B, C and D are somehow interlinked and propose to test if A+B+C=D.
You have gone through numerous literature sources and only find that others have tested the relationship of (A to D) and (B to D), but little is known about C or its relationship to A, B and D.
Thus, in your introduction, you are expected to give an overview/summary of the following:
The ability to accomplish all these in a manner that is convincing to most readers, especially within a given word limit or restricted space, is a skill that scientific/medical writers could hone over time, with enough practice and exposure.
Seek guidance from your senior colleagues where needed and reach out to others who you feel may be able to help you.
Give each manuscript your best effort, and remember that even the most expert writers are all actively learning.