Learn Dutch: Een verfrissende geur is goed voor de bloeddruk. - A refreshing scent is good for the blood pressure.

Because geur is a de-word (de geur). Adjectives before singular de-words always take an -e ending, regardless of whether you use de or een. The base word here is the present participle verfrissend (refreshing), which acts just like a regular adjective.

Could I use the word stank instead of geur here?

Only if you wanted to say a bad smell is good for blood pressure! Geur is neutral or positive (scent/smell), whereas stank specifically means a stench or foul odor. Since a refreshing smell is positive, geur is the right choice.

Why is bloeddruk written as one word?

In Dutch, compound nouns are glued together into a single word without spaces. It combines bloed (blood) and druk (pressure). The article of a compound noun always comes from its final part, which is why it is a de-word (from de druk).

How could I express this same idea using the maken dat (causes to) structure from this lesson?