Which psychotropic medication is administered based on an individualized dosage according to blood levels of the drug?

Reasoning: Choice A reason: Thorazine (chlorpromazine) is a first-generation typical antipsychotic that acts primarily by blocking dopamine D2 receptors in the mesolimbic and mesocortical pathways. While it has a defined dosage range, its administration is not routinely guided by therapeutic drug monitoring of serum blood levels. Clinical response and tolerability, rather than measured serum concentrations, typically drive dosage adjustments, making it distinct from drugs requiring individualized blood-level-based dosing protocols. Choice B reason: Clozapine is an atypical antipsychotic with a complex receptor-binding profile, including antagonism at dopamine D4, serotonin 5-HT2A, and muscarinic receptors. Although plasma levels of clozapine can be monitored in specific clinical scenarios such as non-response or toxicity, routine therapeutic drug monitoring is not the standard method used to individualize its dosage. The primary safety monitoring concern for clozapine is agranulocytosis, tracked via absolute neutrophil count (ANC), not serum drug levels as a primary dosing tool. Choice C reason: Lithium carbonate is a mood-stabilizing agent used in the management of bipolar disorder, particularly in preventing manic and depressive episodes. It has a narrow therapeutic index, with serum therapeutic levels typically maintained between 0.6 and 1.2 mEq/L for maintenance and up to 1.5 mEq/L for acute mania. Dosage is individualized and continuously adjusted based on regular serum blood level monitoring, renal function, hydration status, and sodium intake, as toxicity can occur with levels exceeding 1.5 mEq/L, making blood-level-based dosing the defining pharmacological feature of this drug. Choice D reason: Alprazolam is a benzodiazepine that exerts its anxiolytic effect by potentiating the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) at the GABA-A receptor complex, enhancing chloride ion conductance. Its dosing is individualized based on clinical response, degree of anxiety, age, hepatic function, and tolerance, rather than routine measurement of serum drug levels. Therapeutic drug monitoring is not a standard practice for alprazolam in routine clinical management.