The responder is initialized with a pre-shared long-term static key, which is assumed to be pre-authenticated out of band by the initiator.
Message A, sent by the initiator, does not benefit from sender authentication and does not provide message integrity. It could have been sent by any party, including an active attacker. Message contents benefit from message secrecy and some forward secrecy: the compromise of the responder's long-term private keys, even at a later date, will lead to message contents being decrypted by the attacker. 0,2