Final answer:
The PSA nerve block is the best injection to anesthetize the max molars of a 5 y/o child while avoiding side effects.
Step-by-step explanation:
The best injection to anesthetize the max molars of a 5-year-old child while avoiding side effects is the PSA nerve block.
PSA (Posterior Superior Alveolar) nerve block is effective in numbing the maxillary molars without affecting the lower teeth or lip.
It is important to choose this option to minimize potential side effects of other injections such as ASA nerve block or infiltrations.