Final answer:
The most helpful diagnostic test for the 17-year-old patient with signs of a possible drug overdose is serum creatinine kinase because it can quickly diagnose rhabdomyolysis, a condition that requires urgent treatment.
Step-by-step explanation:
The 17-year-old patient presenting with agitation, hallucinations, a seizure, fever (103 degrees F), hypertension (BP 140/90), tachycardia (pulse 120), and normal respirations is showing signs of a possible drug overdose or adverse drug reaction. Given his hyperpyrexia (extremely high fever), which can indicate a severe reaction such as serotonin syndrome or neuroleptic malignant syndrome, and potential for rhabdomyolysis following a seizure, the diagnostic study that would be most helpful in managing the patient is likely to be serum creatinine kinase (D). An elevated serum creatinine kinase level can indicate rhabdomyolysis, which is a serious complication that can arise from drug overdose and needs to be addressed promptly.
A drug screen (A) can also be helpful in identifying the substances involved, but it might not change immediate management. A complete blood count (C) and urine dipstick (B) may offer additional information but are less likely to influence immediate treatment decisions when compared to the urgency of diagnosing rhabdomyolysis.
The most helpful diagnostic study in managing this patient is serum creatinine kinase, as it can indicate rhabdomyolysis, which is an urgent and treatable condition. While a drug screen might identify the substances used, it is less critical for immediate management.