A 44-year-old male with schizophrenia has been taking Haloperidol since his diagnosis at age 24. The patient has recently experienced "jerky" movement of his arms as well as involuntary movements of his face. Which of the following options is likely the underlying cause?

A. Upregulation of dopamine 2 receptors: Long-term use of typical antipsychotics such as haloperidol blocks dopamine D2 receptors in the basal ganglia. Chronic blockade causes the brain to compensate by increasing (upregulating) the number and sensitivity of these receptors, leading to involuntary, repetitive movements known as tardive dyskinesia. B. Upregulation of serotonin 2A receptors: Serotonin 2A receptor changes are more commonly associated with atypical antipsychotics or serotonergic drugs, not with the movement disorders caused by dopamine receptor blockade. They do not play a primary role in tardive dyskinesia. C. Downregulation of serotonin 2A receptors: Decreased serotonin receptor activity may affect mood and cognition but is not directly linked to extrapyramidal side effects. Tardive dyskinesia stems from dopaminergic rather than serotonergic receptor changes. D. Downregulation of dopamine 2 receptors: Downregulation would reduce dopaminergic signaling and potentially lessen movement-related symptoms. However, in chronic haloperidol therapy, the opposite occurs—dopamine receptors become hypersensitive and increase in number, producing the abnormal movements seen in tardive dyskinesia.