Here is the sentence:
The hike through the canyon exhausted Kim. The steep incline made her thigh
muscles throb, __________, and the relentless sun burned her skin.
The blank requires a short main clause. You chose the heavy pack caused her shoulders to ache, which matches the steep incline made her thigh muscles throb and the relentless sun burned her skin.
The structure is thus parallel.
Nice work, grammar cowpoke!