An 18-month-old male is brought in for routine care. He rolled over at
4 months, sat alone at 9 months, and pulled to a stand at 14 months. He still does not walk alone. He plays with toys, but his mother states that he is strong-willed and will not follow rules. He does not speak, but he babbles. He was born at term and was weight appropriate. He has been healthy and had all immunizations. He is at the 50th percentile for height, head circumference, and weight, but his features are dysmorphic. He has two cafe au lait spots. He will not engage in play but takes a ball and wants to keep it. He plays with blocks and stacks them. He recovers the ball when covered with his blanket. He does not speak at any time during the evaluation. He tries to get the reflex hammer and hesitates when told "no" but still takes it. He relinquishes it on the second request. What test would be most appropriate?