Using the Stun-gun Wristwatch and Voice-Changing Bowtie, Conan reveals through Kogoro that the true murderer is in fact Yasuko Kawakami. He showcases the trick which the culprit used in order to not be seen by Ran and Yayoi in the kitchen, which include moving the flower pot and having Yayoi open the refrigerator which obstructs the view of the see-through door, enabling Yasuko to sneak past the kitchen, up the stairs and into Katsuko's room in order to murder her.
As evidence he points out the shampoo which dripped off Yasuko's body, since she murdered Katsuko while pretending to shower, as well as a wound on her left arm which she inflicted on herself unintentionally by cutting herself on a nail, which hangs on the wall and can be analyzed to reveal the blood on it to be her.
Yasuko's Motive for her murder was Katsuko's tap dancing.
Katsuko's room was upstairs while Yasuko's room is on the ground floor. As Yasuko pleads for Katsuko not to dance in her room, She responded Coldly that tap-dancing in her room is fun, since she feels like she is "stepping" on her brain, then she puts out some insults and tells her to be patient. This led to her death.