Final answer:
A mutation resulting in a defective enzyme A would lead to an accumulation of A and no production of B and C.
Step-by-step explanation:
A mutation resulting in a defective enzyme A would lead to an accumulation of A and no production of B and C. This is because a defective enzyme A would not be able to catalyze the reaction necessary for the production of B and C. As a result, A would accumulate while the production of B and C would be inhibited.