Why is Dr.Bledsoe so angry with the narrator?

Showing his naivete, the narrator is amazed when he hears that Bledsoe would have expected him to find excuses, to lie, instead of stopping in the slums or at a brothel. Bledsoe demands to know who told the narrator to drive where he did, shocking the narrator even further. He cries out that he is lying, and calls the narrator "nigger," enraging the narrator by using that word.