Take a look at the item again:
While Dr. Kinser bounced down the stairs
(A) and strode through the
hallway. (B)
A brightly colored tie streamed over his
shoulder, (C) flapping
in his wake.
You wanted to do this:
While Dr. Kinser bounced down the stairs, and
strode through the hallway. A brightly colored tie streamed over his shoulder,
flapping in his wake.
Adding a comma after stairs is an error. When you have two items joined by and—bounced and strode in this example—do not use a comma with and.
You might want to review the rules.
Go back to the sentence to try again.