"Why do you call me good?" Jesus answered. "No one is good--except God alone.
I am reading an argument (by Arthur Drews) for scribal accidental interpolation in this point in Mark. The original reading would be found partly in Matthew 19:17:
"Why do you ask me about what is good?" Jesus replied. "There is only One who is good. If you want to enter life, keep the commandments."
Since the original answer was about what was good doing:
"Why do you ask me about what is good?" Jesus replied. "There is only a thing who is good. If you want to enter life, keep the commandments."
"A thing" (neuter) was translated as "one" (male gender) and it moved the scribe to ask who could be called good. So the scribal note in the margin of text ("god is only good") was inserted accidentally in the text as it is now in Mark.