Stool Sample is a challenge to discover the five piles of borok dung in Hunter's Grotto. The piles look similar to loot piles, but have two skulls and a leg sticking out of it instead of an assortment of bones, and have brown steam wafting off of it as opposed to loot piles' green.
The first pile of borok dung can be found immediately at the bottom of the stairs outside the tunnel leading to the western area of Hunter's Grotto; the second pile is immediately west of it, on the far side of the clearing past Nakayama's animal trap.
The third and fourth piles of dung are in the clearing to the north, beyond the river; one should be visible just outside of a borok stump for those crossing the river, and the other is just south of the large tree to the west.
The final pile of dung is up the river, on a small rock outcropping. To reach it one must return to the river and head upstream, climbing the rapids to the waterfall. To the left there will be a gap between the cliff face and a rock. The last pile is along this path and to the left around a rock.