Obtained: Speak to Clenching Mr. Dougen (blue mascot in front of Dougen Saurus stand) in the Daitama Observatory.
Obtain the anti-diarrhea medicine from a pharmacy between Hee Ho Mart and Tokuseido (book store) for 1,700 yen. Hurry back to Daitama and hand over the desperately needed medicine!
Reward: Promised Quartet