Which of the following is correct regarding the metabolism of phenytoin as shown below? (Select all that apply)
The process that takes place in the step A in the figure above is mediated by CYP3A4 enzyme
The metabolite which results from the step A above is usually more water soluble than the metabolite that is produced from the step B above
The letter B in the figure above represents the phase 1 reaction step in the metabolism of phenytoin
For the step A in the figure above, preparatory reaction such as oxidation, reduction or hydrolysis usually take place
The letter A in the figure above represents the phase 1 reaction step in the metabolism of phenytoin
Correct Answer : D,E
Phenytoin is an anticonvulsant that undergoes complex hepatic metabolism, primarily via the cytochrome P450 system, specifically the CYP2C9 and CYP2C19 isoenzymes. Metabolism typically involves a Phase 1 reaction (functionalization) followed by a Phase 2 reaction (conjugation). The goal of these enzymatic processes is to increase the hydrophilicity of the lipophilic drug to facilitate renal excretion and prevent systemic toxicity from drug accumulation.
Rationale:
A. Step A in phenytoin metabolism involves the hydroxylation of the phenyl ring to form 4-hydroxyphenytoin. This specific oxidative process is primarily mediated by CYP2C9 and CYP2C19, rather than CYP3A4. Identifying the correct isoenzyme is vital because genetic polymorphisms in CYP2C9 can significantly alter the rate of phenytoin clearance and clinical response.
B. The metabolite from Step A is a hydroxylated product, which is a Phase 1 metabolite. Step B typically represents glucuronidation, a Phase 2 reaction that attaches a large sugar moiety to the molecule. Consequently, the Phase 2 metabolite is significantly more water soluble than the Phase 1 metabolite, contrary to the statement in the choice.
C. The letter B represents the conjugation step, which is defined as a Phase 2 reaction. During this stage, the molecule is coupled with an endogenous substrate like glucuronic acid to enhance its polarity. Phase 1 reactions (functionalization) must generally occur before these Phase 2 pathways can proceed to facilitate the final elimination of the drug.
D. Step A involves the introduction of a functional group (-OH) through an oxidative process. This is the definition of a preparatory reaction in drug metabolism, which includes oxidation, reduction, or hydrolysis. These reactions prepare the lipophilic phenytoin molecule for subsequent conjugation, making it a critical initial step in the detoxification and elimination pathway.
E. Step A correctly represents the Phase 1 reaction, which is the initial stage of biotransformation. In this phase, enzymes modify the drug's structure to make it more polar or to provide a "handle" for the next phase. Phenytoin's conversion to its parahydroxylated metabolite is a classic example of Phase 1 aromatic hydroxylation in the liver.
Nursing Test Bank
Naxlex Comprehensive Predictor Exams
Related Questions
Correct Answer is A
Explanation
Step 1 is to identify the LDL values for males
Male values: 195, 150, 170, 150, 160
Step 2 is to calculate the Mean
(195 + 150 + 170 + 150 + 160) ÷ 5
195 + 150 = 345
345 + 170 = 515
515 + 150 = 665
665 + 160 = 825
825 ÷ 5 = 165
Result at this step = 165
Step 3 is to find the Median
Arrange values: 150, 150, 160, 170, 195
The middle value is 160
Result at this step = 160
Answer: A
Correct Answer is A
Explanation
Albuterol is a sympathomimetic bronchodilator categorized as a short-acting beta-2 agonist (SABA). It selectively stimulates beta-2 receptors in the lungs to induce smooth muscle relaxation via adenylate cyclase activation. This increases intracellular cAMP levels, effectively reversing acute bronchoconstriction in asthma or COPD patients. Side effects commonly include tachycardia, tremors, and palpitations due to transient stimulation of beta-1 and skeletal muscle receptors.
Rationale:
A. ProAir HFA is the correct answer as it is a brand-name formulation of albuterol sulfate. It utilizes a metered-dose inhaler with hydrofluoroalkane propellant to deliver the medication directly to the bronchioles. This product is the gold standard for providing rapid relief of respiratory distress. It is frequently prescribed for exercise-induced bronchospasm and acute asthma exacerbations.
B. Primatene Mist contains epinephrine as the active ingredient, which is a non-selective alpha and beta-adrenergic agonist. While it provides bronchodilation, it lacks the beta-2 selectivity found in albuterol and has a higher risk of systemic cardiovascular stimulation. It is an over-the-counter medication and is not a formulation of albuterol.
C. Asthmanefrin contains racepinephrine, which is a racemic mixture of epinephrine. It is delivered via an atomizer but is not considered a SABA like albuterol due to its non-selective adrenergic profile. It is primarily used for temporary relief of bronchial asthma symptoms but does not contain the specific albuterol molecule.
D. Xopenex HFA contains levalbuterol, which is the (R)-enantiomer of albuterol. While structurally related, it is a purified form that removes the (S)-isomer, which is thought to be clinically inactive or potentially pro-inflammatory. Therefore, while it acts similarly, the active ingredient is levalbuterol, not racemic albuterol, making this choice technically incorrect.
E. Xopenex Concentrate is the nebulized solution form of levalbuterol. Like the HFA version, it specifically provides the single-isomer levalbuterol for inhalation therapy. It is used for patients who cannot effectively use a handheld inhaler or require high-dose therapy in a clinical setting. It does not contain standard racemic albuterol.
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