Which movement disorder involves sustained muscle contractions and can develop within the first week of treatment?

The movement disorder characterized by sustained muscle contractions is dystonia. This condition is often associated with the use of antipsychotic medications, particularly when a patient has recently started treatment or had their dosage increased. Dystonia can manifest as abnormal postures, twisting movements, and pain due to prolonged muscle contractions. These symptoms can indeed develop within the first week of treatment, which is a key distinguishing feature of this disorder.

The other disorders listed do have movement-related symptoms, but they differ significantly in their presentation and timing. For instance, akathisia primarily involves a feeling of inner restlessness and an uncontrollable need to be in constant motion, rather than sustained muscle contractions. Parkinsonism includes symptoms such as tremors, rigidity, and bradykinesia but typically develops over a longer period after initiating treatment. Tardive dyskinesia is characterized by repetitive, involuntary movements, often arising after prolonged exposure to antipsychotic medications, rather than shortly after treatment begins. Thus, the distinct timing and nature of muscle contractions in dystonia make it the correct answer in this context.