The key from Ratthew is used in Room 5 , where you remove the cat poster to reveal a secret lock , then open it with the key. That unlocks a hidden clue tied to Ratthew’s lore, including a picture of Retina, his sister.

Where to use it

  • Go to Room 5.
  • Look for the cat poster.
  • Rip/remove the poster to expose the secret lock.
  • Use the key on that lock.

When you get it

  • In the update/lore sequence, Ratthew gives you the key after the conversation finishes and he leaves you a note with it.
  • Recent videos about the update and secret key were posted on June 25–26, 2026 , so this is part of the current Animal Hospital update content.

What it unlocks

  • The lock reveals a hidden image connected to Retina , Ratthew’s sister.
  • It’s part of the game’s secret lore rather than a normal treatment-room item.

TL;DR: Use Ratthew’s key in Room 5 behind the cat poster to open the secret lock.