Writing the intro
About 75 words. More important than its size suggests.
By the time you sit down to write your introduction, you should already have your diagrams drawn and your analysis paragraphs drafted. The intro is written last-ish: or at least redrafted last: because it needs to set up everything that follows, and you can't set up what you haven't figured out yet.
What goes in it
The introduction has two jobs:
Introduce the economic problem
Describe what the article is about and the economic problem it raises. Use the relevant economic terms naturally as you describe what is happening: don't write a definitions paragraph, just use the terms in context the way you'd use them if you were explaining the situation to someone. The model IA opens by describing how overconsumption of sugary drinks "exemplifies market failure when the market fails to achieve allocative efficiency": terms like demerit goods, market failure, and negative consumption externality appear because the analysis needs them, not to demonstrate vocabulary.
Terms should appear because you need them, not to show you know them.
Explain your key concept
State which concept you're using and explain what it means in this specific context. Don't just name it: say what it means economically and why this article is an example of it. The model IA writes that the market "fails to achieve allocative efficiency, where marginal social benefit equals marginal social cost": the concept is explained in the same sentence it appears, not dropped in as a label. Bold the concept word each time it appears.
This is your first chance to score on Criterion D. A vague sentence like "this article relates to the concept of efficiency" scores nothing. A sentence that shows you understand what the concept means and how it applies to this specific market starts scoring immediately.