This page explains how to start and complete the Medicinal Molduga side quest.
How to Begin the Quest
Read Reach Gerudo Town for a guide to entering Gerudo Town. Once there, talk to Malena in the west corner of town. She talks to a guard in the Barracks in the west corner of town during the day, then talks to a guard in front of Riju's throne room, then goes to bed in the inn at night.
How to Complete the Quest
Malena needs Molduga Guts for medicine. There are four Moldugas in the Gerudo Desert. One is in Toruma Dunes which is to the west of Gerudo Town. One is near the Southern Oasis which is to the south of Gerudo Town. The others are southwest of Gerudo Town, one in the Arbiter's Grounds which are southwest of the Southern Oasis, and one is to the northwest of the Arbiter's Grounds.
To defeat a Molduga, avoid walking too close or it will go underneath you and swallow you. You can try to lure it out from under the sand by throwing a bomb between you and it, then detonating the bomb when the Molduga swallows it. If it launches you into the air, you can use the Paraglider and you can try to shoot it from above. You can also try to fight the Molduga while using a Sand Seal.
Quest Reward
Malena gives you a Gold Rupee.