To send a PGP encrypted message, which piece of information from the recipient must the sender have before encrypting the message?

The recipient's private key is the secret decryption key that must never be shared; if the sender possessed it, the entire security model would be broken.

PGP uses asymmetric (public-key) cryptography where each user has a mathematically linked key pair. The sender must obtain and use the recipient's public key to encrypt the message; this ensures that only the recipient - who holds the corresponding private key - can decrypt and read it. The public key is safe to distribute openly, so the recipient can share it without compromising security.

There is no 'master encryption key' in PGP's decentralized trust model - each user maintains their own key pair and there is no central key authority.

The sender's own public key is used by others to encrypt messages addressed to the sender, not for encrypting messages the sender wishes to send to someone else.