"Draco Abraxas Malfoy, if you think I'm going to believe that, then you have another thing coming." Even Draco's pitiful look as he drank the Skele-Gro didn't get rid of any of Narcissa's irritation at her son's pathetic attempt to lie. She knew how bad the stuff was, but right now she could care less.
He should know better than to try and lie to his mother.
"Tell me the truth, and tell it now. As you so often remind me, you're a grown man — don't make me treat you like a child," Narcissa said angrily. Normally she wasn't so harsh towards Draco, but tonight's events had her emotions all over the place, and anger was the easiest one to settle on. She was a Black, after all, and they usually took refuge in anger. Narcissa usually hid her stronger feelings, but tonight was going to be a different matter.