A man pulled out a gun and fired at least one shot in a waiting area at a hospital in the Bronx on Wednesday night, injuring a nurse and a security guard, the police said. The shooting occurred in the waiting area for the Bronx-Lebanon Hospital Center's emergency room, and detectives were investigating the possibility that the gunman might have been firing at someone he thought was a rival gang member, said Paul J. Browne, the chief spokesman for the New York Police Department. The suspect entered the waiting room, in the Mount Eden section of the Bronx, with a concealed weapon, the police said. About 7 p.m., he fired at least one shot, the police said. Two men were hit: a nurse, 37, in the shoulder, and a security guard, 42, in the lower body, said Lt. John Grimpel, a spokesman for the Police Department. Both were in stable condition and expected to be released.