This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Dental
A mother and her 4 year old son are seated alone in
a reception area with the child staring off into space,
rocking and constantly twisting a strand of hair about
his fingers. Upon entry of another person, the child
begins to beat his fist against the side of his face and
behaves as though he does not hear his mother speaking
to him. This behavior is most characteristic of: 
Select and explain. 
A. An autistic child, B. A mentally retarded child, C. First dental appointment anxieties of a 4 year old child, D. A child with a chronic seizure disorder
A: An autistic child
Answer: A: An autistic child 
Explanation: None