Review the item:
When Jim threw his back out while helping his wife Nancy move the sofa,
he feared the treatment that the doctor would recommend,
a week's worth of bed rest during the nicest week in April would
certainly wreck Jim's golf plans.
For a sentence to be fused, you must have two complete sentences run together with no punctuation. In the example above, a comma joins the two sentences. He feared the treatment that the doctor would recommend is one sentence. A week's worth of bed rest during the nicest week in April would certainly wreck Jim's golf plans is the second sentence.
Thus, the error is a comma splice.
To fix the problem above, you could put a period after recommend and capitalize the A.