There are many special achievements in The Lake House, and "First Aid" is one of them. To unlock this achievement, you need to go to the [Farewell Guests] level, interact with Aldous's lower leg using a fork, remove three pellets, and then close the wound.
How to Unlock the First Aid Achievement in Rusty Lake: The Lake House
First Aid
Emergency Treatment
In the [Farewell Guests] level, use a fork to interact with Aldous's lower leg, remove three pellets, and then close the wound.
Related Guide: How to Unlock the "First Aid" Achievement in The Lake House