A woman in her 70s was seriously injured after an MBTA bus struck her as she walked in Dorchester o Monday morning, authorities said. The woman, who has not been identified, was reportedly hit at about 11:35 a.m. by a No. 41 bus at a busy intersection in Uphams Corner, T officials said. She was transported to Boston Medical Center and remained in critical condition Monday evening, said MBTA spokeswoman Kelly Smith. The driver was taken to a hospital for stress-related ailments, said Joseph O’Connor, superintendent in chief of the MBTA Transit Police.