Final answer:
The schizophrenic man's symptoms are most consistent with Neuroleptic malignant syndrome (NMS), a condition associated with antipsychotic medication use and characterized by fever, muscle rigidity, and autonomic instability.
Step-by-step explanation:
The most likely diagnosis for a schizophrenic man on an inpatient psychiatric unit who develops a fever of 102.3°F (39.1°C), a high white blood cell (WBC) count, unstable pulse and blood pressure, and rigidity in the arms and legs is Neuroleptic malignant syndrome (NMS). NMS is a potentially life-threatening condition typically associated with the use of antipsychotic medications, which are commonly prescribed for schizophrenia. The symptoms of NMS include hyperthermia, altered mental status, muscle rigidity, autonomic instability, and increased serum creatine phosphokinase.
While meningitis is an infection that can cause fever and a stiff neck, it is less likely in this context unless there is a concurrent infection on top of the medication side effects. The symptoms presented don't align with serotonin-specific reuptake inhibitor withdrawal or lithium toxicity, which have different clinical presentations. Phencyclidine (PCP) use is also less likely given the findings and the setting. Therefore, the presentation is most consistent with the features of NMS.