World's Finest Eye Drops
The World's Finest Eye Drops is a quest item that can only be obtained through the Biggoron's Sword sidequest. As the name indicates, these are in fact the finest eye drops in the world and are concocted from the eyes of the Eyeball Frog. However, Eyeball Frogs will spoil quickly, making it difficult to transport the item and utilize it to create the Eye Drops.
Obtaining the World's Finest Eye Drops
At the Lakeside Laboratory, while Link is in the midst of acquiring the Biggoron's Sword, he finds that he needs the Eyeball Frog to create the World's Finest Eye Drops for Biggoron. After receiving it from King Zora XVI at Zora's Domain, he must bring it to the Lake Scientist on horseback for increased speed, as the Eyeball Frog spoils quickly.
The Eye Drops are meant for Biggoron, so Link must travel to Death Mountain Summit as quickly as possible. However, he is not allowed to warp using a warp song. The Eyeball Frog spoils in just four minutes, so he must make haste. The easiest way to accomplish the feat is if Link plays Epona's Song and rides to Death Mountain on horseback. Once the Eye Drops are delivered to Biggoron, he will feel much better and give Link the Claim Check, signifying that he will indeed be working on the Biggoron's Sword.