The preposition is "in front".
B. Macbeth is having a hallucination that he is holding a knife.
The doctor tells Macbeth that his wife is very ill, and he cannot cure her. Macbeth reacts angrily, telling the doctor he cannot be bothered by such matters. This is so different than the start of the play when Macbeth and Lady Macbeth are shown as a close, loving couple. We know when Macbeth no longer cares about his wife that he is headed for tragedy
