Two days after starting an antipsychotic medication, a patient suddenly develops a frightening, sustained spasm that forcibly twists his neck to one side. What is this?

This is acute dystonia: a sudden, involuntary, sustained muscle contraction producing an abnormal and frightening posture, arising early in antipsychotic treatment, exactly as described. It is not a catatonic stereotyped posture, since acute dystonia differs from a catatonic stereotyped posture in its acute onset, clear medication association, and typically spasmodic rather than steadily maintained quality. It is not waxy flexibility, which involves a smooth, plastic resistance felt throughout passive movement rather than a spontaneous, sudden spasm. And it is not akathisia, which is a subjective, distressing inner restlessness driving movement rather than a sustained muscular contraction.