What is a defining characteristic of an acute dystonic reaction?

An acute dystonic reaction is primarily characterized by sustained muscle contractions, which can lead to abnormal postures and movements. This form of extrapyramidal side effect is often triggered by the use of antipsychotic medications or other drugs that influence dopamine pathways in the brain. The muscle contractions can affect various muscle groups, leading to symptoms such as torticollis (twisted neck), oculogyric crisis (involuntary eye movement), and other involuntary spasms. Recognizing these muscle contractions is crucial for prompt diagnosis and treatment of the condition, as they can develop rapidly and may require immediate intervention, such as the administration of anticholinergic medications.

Other symptoms listed in the options, such as persistent coughing, flat affect, and nausea and vomiting, may be associated with various other medical conditions or side effects but do not specifically define an acute dystonic reaction. Thus, the hallmark feature of muscle contractions distinguishes this reaction clearly from those other symptoms.