Pharmacology Quiz: Antiepileptics
20 questions · exam conditions
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AntiepilepticsQuestion 1 of 20

A 6-month-old infant with infantile spasms has failed conventional therapies. Treatment with vigabatrin is being considered. The parents are counseled about a significant, irreversible toxicity associated with this drug.

What specific long-term monitoring is mandated for all patients receiving vigabatrin?

Monthly complete blood counts for aplastic anemia.
Serial echocardiograms for cardiac fibrosis.
Annual renal ultrasounds for nephrocalcinosis.
Regular ophthalmologic examinations for visual field defects.
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Pharmacology Quiz

Pharmacology Quiz: Antiepileptics

Practice Antiepileptics in Pharmacology with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

What this quiz covers

This quiz focuses on Antiepileptics, giving you a quick way to practice the rules, question types, and explanations that matter most for Pharmacology.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

All questions

Question 1

A 6-month-old infant with infantile spasms has failed conventional therapies. Treatment with vigabatrin is being considered. The parents are counseled about a significant, irreversible toxicity associated with this drug.

What specific long-term monitoring is mandated for all patients receiving vigabatrin?

  1. Monthly complete blood counts for aplastic anemia.
  2. Serial echocardiograms for cardiac fibrosis.
  3. Annual renal ultrasounds for nephrocalcinosis.
  4. Regular ophthalmologic examinations for visual field defects. (correct answer)
Explanation: When you encounter questions about antiepileptic drug monitoring, focus on each medication's unique toxicity profile and required surveillance protocols. Vigabatrin carries a serious, dose-related risk of bilateral concentric visual field defects that can occur in up to 40% of patients. These defects are typically irreversible and asymptomatic until severe, making regular ophthalmologic monitoring absolutely essential. The visual field loss results from vigabatrin's toxicity to retinal cone cells, particularly in the peripheral retina. This is why answer D is correct – all patients on vigabatrin require baseline ophthalmologic examination before treatment and regular follow-up visual field testing every 6 months. A is incorrect because vigabatrin does not cause aplastic anemia or significant hematologic toxicity requiring routine blood count monitoring. B is wrong as vigabatrin is not associated with cardiac fibrosis – you might be thinking of medications like methysergide or ergot alkaloids. C is incorrect because nephrocalcinosis is not a recognized complication of vigabatrin therapy; this toxicity is more associated with medications like topiramate or carbonic anhydrase inhibitors. The visual field defects are particularly concerning because they're irreversible even after drug discontinuation, though they typically don't progress once the medication is stopped. This irreversible nature is why the parents in this case are being counseled about "significant, irreversible toxicity." Study tip: For antiepileptic drugs, memorize the major toxicity requiring monitoring for each: vigabatrin (visual fields), carbamazepine (blood counts), valproic acid (liver function), and phenytoin (gingival hyperplasia/osteomalacia).

Question 2

A 29-year-old woman is being treated with levetiracetam for focal seizures. During a follow-up visit, she reports feeling persistently irritable, agitated, and emotionally labile since starting the medication, which is affecting her relationships.

This patient's symptoms are a well-documented side effect of levetiracetam. What is the most appropriate initial management step?

  1. Supplement with pyridoxine (Vitamin B6) and monitor for improvement. (correct answer)
  2. Add a low dose of an antipsychotic agent to manage the agitation.
  3. Immediately discontinue levetiracetam and switch to lamotrigine.
  4. Increase the dose of levetiracetam, as this may be a paradoxical effect.
Explanation: When encountering psychiatric side effects from antiepileptic drugs, your first consideration should be whether there's a safe, evidence-based intervention that can mitigate these effects without compromising seizure control. Levetiracetam commonly causes behavioral side effects including irritability, agitation, and mood changes in approximately 10-15% of patients. Research has shown that pyridoxine (Vitamin B6) supplementation can effectively reduce these psychiatric symptoms. The mechanism likely involves pyridoxine's role as a cofactor in neurotransmitter synthesis, particularly GABA and serotonin, which may counteract levetiracetam's effects on mood regulation. This makes option A the most appropriate initial step. Option B is incorrect because adding antipsychotics introduces unnecessary polypharmacy and potential side effects when a simpler, safer intervention exists. Option C represents poor seizure management - abruptly discontinuing an effective antiepileptic drug risks breakthrough seizures, and switching medications should only be considered after conservative measures fail. The tapering process for levetiracetam and titration of lamotrigine would also leave the patient vulnerable during the transition period. Option D shows a fundamental misunderstanding - increasing the dose would likely worsen the behavioral symptoms, as these are dose-related side effects, not paradoxical reactions. Study tip: For antiepileptic drug side effects, remember the hierarchy: first try conservative interventions (like vitamin supplementation for levetiracetam's psychiatric effects), then consider dose adjustments, and only switch medications as a last resort due to seizure risks during transitions.

Question 3

A physician in the emergency department administers an intravenous drug to a patient in status epilepticus. The drug acts by binding to an allosteric site on the GABA-A receptor, which increases the frequency of chloride channel opening in the presence of GABA.

Which antiepileptic drug was most likely administered?

  1. Phenobarbital
  2. Lorazepam (correct answer)
  3. Tiagabine
  4. Valproic acid
Explanation: This question tests the specific mechanistic distinction between benzodiazepines and barbiturates at the GABA-A receptor. Benzodiazepines, like lorazepam, increase the frequency of chloride channel opening. Barbiturates, like phenobarbital, increase the duration of channel opening. Tiagabine inhibits GABA reuptake, and valproic acid has multiple mechanisms, including increasing GABA synthesis and inhibiting its degradation, but neither primarily alters channel opening frequency. Lorazepam is a first-line agent for status epilepticus.

Question 4

A 45-year-old woman of Korean descent is diagnosed with trigeminal neuralgia. Her physician plans to initiate treatment with carbamazepine. Her medical history is otherwise unremarkable.

Which of the following is the most critical screening test to perform before initiating carbamazepine in this patient?

  1. Complete blood count with differential
  2. HLA-B*1502 genetic testing (correct answer)
  3. Serum sodium and creatinine levels
  4. Baseline electrocardiogram (ECG)
Explanation: Carbamazepine carries a significant risk of Stevens-Johnson syndrome (SJS) and toxic epidermal necrolysis (TEN) in patients with the HLA-B*1502 allele. This allele is most prevalent in individuals of Asian descent. The FDA recommends screening for this allele in this patient population before starting carbamazepine. While monitoring CBC for aplastic anemia and serum sodium for hyponatremia are important during treatment, the pre-treatment genetic screening is most critical to prevent a potentially fatal hypersensitivity reaction.

Question 5

A 40-year-old woman is taking an enzyme-inducing antiepileptic drug for seizure control. She is also taking a combined oral contraceptive pill (ethinyl estradiol/norethindrone) for contraception.

Which of the following best describes the expected interaction and the most appropriate management advice?

  1. The antiepileptic will increase contraceptive levels, requiring a lower contraceptive dose to avoid toxicity.
  2. The antiepileptic will decrease contraceptive efficacy, necessitating use of a barrier method or alternative contraception. (correct answer)
  3. The oral contraceptive will induce the metabolism of the antiepileptic, leading to breakthrough seizures.
  4. There is no significant interaction between these classes of drugs, and no changes are needed.
Explanation: Enzyme-inducing antiepileptics (e.g., phenytoin, carbamazepine, phenobarbital, topiramate at doses >200mg/day) increase the hepatic metabolism of the estrogen and progestin components of oral contraceptives. This leads to lower hormone levels and a significant risk of contraceptive failure. Patients should be counseled to use alternative or additional contraceptive methods, such as an IUD or depot medroxyprogesterone acetate, which are less affected by this interaction.

Question 6

A 22-year-old patient with bipolar disorder is treated with valproic acid. To augment treatment for seizure control, lamotrigine is added, starting at 25 mg daily. Two weeks later, the patient develops a diffuse, morbilliform rash on her trunk and arms.

What is the most likely pharmacokinetic mechanism underlying this adverse reaction?

  1. Valproic acid induces CYP2C9, leading to the formation of a reactive lamotrigine metabolite.
  2. Lamotrigine displaces valproic acid from albumin, increasing free valproic acid concentration.
  3. Valproic acid inhibits UGT enzymes, significantly reducing the clearance of lamotrigine. (correct answer)
  4. Both drugs compete for active tubular secretion in the kidney, prolonging their half-lives.
Explanation: Valproic acid is a broad-spectrum enzyme inhibitor, notably of the UDP-glucuronosyltransferase (UGT) enzymes responsible for metabolizing lamotrigine. This inhibition doubles the half-life of lamotrigine, leading to higher-than-expected serum concentrations. Rapid dose titration or standard starting doses of lamotrigine in the presence of valproic acid significantly increases the risk of serious skin rashes, including SJS. Therefore, when these drugs are used concurrently, a much slower titration and lower target dose of lamotrigine are required.

Question 7

A patient stabilized on a maintenance dose of carbamazepine for several months reports a return of focal seizures over the past two weeks despite confirming perfect adherence to his regimen. He has not started any new medications or herbal supplements.

What is the most likely pharmacological explanation for this patient's breakthrough seizures?

  1. Development of pharmacodynamic tolerance at the sodium channel receptor site.
  2. Progressive decrease in drug absorption due to gastrointestinal P-glycoprotein upregulation.
  3. Autoinduction of hepatic CYP3A4 enzymes, leading to increased carbamazepine clearance. (correct answer)
  4. Saturation of plasma protein binding sites, resulting in a lower total drug concentration.
Explanation: Carbamazepine is a potent inducer of its own metabolism, primarily through the CYP3A4 enzyme system. This process, known as autoinduction, begins after a few days of therapy and can take 3-5 weeks to complete. The result is an accelerated clearance and a shorter half-life of the drug, which can lead to subtherapeutic levels and breakthrough seizures even with consistent dosing. This often necessitates an upward dose adjustment after the initial weeks of therapy.

Question 8

A patient with epilepsy and a history of calcium oxalate kidney stones requires a new antiepileptic medication. The physician wishes to avoid agents that might exacerbate stone formation.

Which antiepileptic drug should be used with the most caution in this patient due to its known risk of causing metabolic acidosis and promoting nephrolithiasis?

  1. Topiramate (correct answer)
  2. Levetiracetam
  3. Gabapentin
  4. Lamotrigine
Explanation: When evaluating antiepileptic drugs for patients with kidney stone history, you need to consider each medication's metabolic effects and potential to alter urine chemistry in ways that promote stone formation. Topiramate (A) is the correct answer because it's a carbonic anhydrase inhibitor that causes metabolic acidosis and significantly increases nephrolithiasis risk. By inhibiting carbonic anhydrase in the kidneys, topiramate reduces hydrogen ion secretion and bicarbonate reabsorption, leading to metabolic acidosis. This acidosis increases urinary citrate loss and creates an alkaline urine environment that promotes calcium phosphate precipitation. Additionally, topiramate directly increases calcium excretion. These combined effects make it particularly dangerous for patients with existing stone disease. Levetiracetam (B) has an excellent renal safety profile with minimal metabolic effects and no known association with kidney stones. Gabapentin (C) is also renally safe, though it requires dose adjustment in kidney disease, it doesn't promote stone formation. Lamotrigine (D) occasionally causes metabolic acidosis but lacks the strong association with nephrolithiasis seen with topiramate. The key distinction is that topiramate uniquely combines carbonic anhydrase inhibition with direct effects on calcium handling, creating a "perfect storm" for stone formation through multiple mechanisms: metabolic acidosis, increased calcium excretion, decreased citrate excretion, and alkaline urine pH. Study tip: Remember that carbonic anhydrase inhibitors (topiramate, acetazolamide) are the antiepileptic drugs most strongly associated with kidney stones. When you see kidney stone history in pharmacology questions, immediately flag any carbonic anhydrase inhibitor as high-risk.

Question 9

A patient is prescribed pregabalin for post-herpetic neuralgia. The patient asks how the medication works to relieve their pain.

Which of the following best describes the primary mechanism of action of pregabalin?

  1. It directly binds to and activates GABA-A and GABA-B receptors in the spinal cord.
  2. It inhibits the GABA-transaminase enzyme, leading to increased synaptic GABA levels.
  3. It blocks the reuptake of GABA from the synapse via the GAT-1 transporter.
  4. It binds to the alpha-2-delta subunit of presynaptic voltage-gated calcium channels. (correct answer)
Explanation: When encountering questions about gabapentinoids like pregabalin, remember that despite their name similarity to GABA, these drugs don't actually work through GABA mechanisms. This is a common source of confusion on pharmacology exams. Pregabalin works by binding specifically to the alpha-2-delta subunit of presynaptic voltage-gated calcium channels (VGCC). This binding reduces calcium influx into nerve terminals, which decreases the release of excitatory neurotransmitters like glutamate and substance P. By dampening this excitatory signaling, pregabalin effectively reduces neuropathic pain transmission, making it highly effective for conditions like post-herpetic neuralgia. Choice A is incorrect because pregabalin does not bind to or activate GABA receptors directly. While it may indirectly influence GABAergic tone, this isn't its primary mechanism. Choice B describes vigabatin's mechanism, which irreversibly inhibits GABA-transaminase to increase GABA levels. Choice C describes the mechanism of tiagabine, which blocks GABA reuptake via the GAT-1 transporter. The correct answer is D because pregabalin's therapeutic effect stems from its selective binding to the alpha-2-delta subunit of presynaptic VGCCs, reducing excitatory neurotransmitter release. Study tip: Remember that gabapentinoids (pregabalin and gabapentin) are "GABA imposters" – they're named for GABA but work through calcium channels, not GABA systems. Focus on calcium channel modulation when you see these drugs mentioned, especially for neuropathic pain conditions.

Question 10

A 7-year-old child with childhood absence epilepsy has been treated with ethosuximide for 3 months with appropriate dose titration. However, the child continues to have frequent staring spells, confirmed by EEG. The neurologist decides to switch to a different agent.

Which of the following antiepileptics would be an appropriate alternative that works through a primary mechanism different from ethosuximide?

  1. Valproic acid
  2. Clonazepam
  3. Lamotrigine (correct answer)
  4. Zonisamide
Explanation: Ethosuximide's primary mechanism is the blockade of T-type calcium channels in thalamic neurons, which is specific for absence seizures. Lamotrigine is also effective for absence seizures but its primary mechanism is blocking voltage-gated sodium channels. Valproic acid is effective but also blocks T-type calcium channels (in addition to other mechanisms), so it does not represent a completely different primary mechanism. Clonazepam (a benzodiazepine) works on GABA-A receptors but is generally used as a second- or third-line agent due to tolerance and sedation. Zonisamide has weak T-type calcium channel blocking activity.

Question 11

A 55-year-old man on long-term phenytoin therapy for post-traumatic epilepsy is found to have a low bone mineral density on a DEXA scan. His serum calcium is low, phosphate is low, and alkaline phosphatase is elevated.

The patient's bone condition is most likely a consequence of which effect of phenytoin?

  1. Direct inhibition of osteoblast activity and function.
  2. Induction of hepatic enzymes that accelerate Vitamin D catabolism. (correct answer)
  3. Increased urinary calcium excretion due to drug-induced nephropathy.
  4. Chelation of dietary calcium in the gastrointestinal tract.
Explanation: Phenytoin is a potent inducer of cytochrome P450 enzymes in the liver. This induction increases the metabolism of Vitamin D (both D2 and D3) into inactive metabolites. The resulting Vitamin D deficiency impairs intestinal calcium and phosphate absorption, leading to secondary hyperparathyroidism and, over time, osteomalacia (demineralization of bone), which presents with the laboratory findings described. Patients on long-term therapy with enzyme-inducing antiepileptics often require Vitamin D and calcium supplementation.

Question 12

A 45-year-old woman of Korean descent is diagnosed with trigeminal neuralgia. Her physician plans to initiate treatment with carbamazepine. Her medical history is otherwise unremarkable.

Which of the following is the most critical screening test to perform before initiating carbamazepine in this patient?

  1. Complete blood count with differential
  2. HLA-B*1502 genetic testing (correct answer)
  3. Serum sodium and creatinine levels
  4. Baseline electrocardiogram (ECG)
Explanation: Carbamazepine carries a significant risk of Stevens-Johnson syndrome (SJS) and toxic epidermal necrolysis (TEN) in patients with the HLA-B*1502 allele. This allele is most prevalent in individuals of Asian descent. The FDA recommends screening for this allele in this patient population before starting carbamazepine. While monitoring CBC for aplastic anemia and serum sodium for hyponatremia are important during treatment, the pre-treatment genetic screening is most critical to prevent a potentially fatal hypersensitivity reaction.

Question 13

A patient with refractory partial-onset seizures is started on perampanel. The prescriber counsels the patient and their family about a specific black box warning associated with this medication.

Given perampanel's mechanism as a non-competitive AMPA receptor antagonist, which of the following adverse effects is the subject of its black box warning and requires careful monitoring?

  1. Fatal hepatotoxicity and liver failure.
  2. Irreversible vision loss and retinal damage.
  3. Aplastic anemia and agranulocytosis.
  4. Serious or life-threatening psychiatric and behavioral reactions. (correct answer)
Explanation: Perampanel carries a black box warning for serious psychiatric and behavioral reactions, including aggression, hostility, irritability, anger, and homicidal ideation. These effects are thought to be related to its mechanism of blocking the AMPA glutamate receptor, a key component of excitatory neurotransmission. Patients and caregivers must be warned to monitor for these changes, especially during dose titration and at higher doses. The other options are black box warnings for other antiepileptics (felbamate/valproic acid, vigabatrin, and carbamazepine/felbamate, respectively).

Question 14

A patient stabilized on a maintenance dose of carbamazepine for several months reports a return of focal seizures over the past two weeks despite confirming perfect adherence to his regimen. He has not started any new medications or herbal supplements.

What is the most likely pharmacological explanation for this patient's breakthrough seizures?

  1. Development of pharmacodynamic tolerance at the sodium channel receptor site.
  2. Progressive decrease in drug absorption due to gastrointestinal P-glycoprotein upregulation.
  3. Autoinduction of hepatic CYP3A4 enzymes, leading to increased carbamazepine clearance. (correct answer)
  4. Saturation of plasma protein binding sites, resulting in a lower total drug concentration.
Explanation: Carbamazepine is a potent inducer of its own metabolism, primarily through the CYP3A4 enzyme system. This process, known as autoinduction, begins after a few days of therapy and can take 3-5 weeks to complete. The result is an accelerated clearance and a shorter half-life of the drug, which can lead to subtherapeutic levels and breakthrough seizures even with consistent dosing. This often necessitates an upward dose adjustment after the initial weeks of therapy.

Question 15

A 38-year-old patient initiated on topiramate for migraine prophylaxis reports significant cognitive side effects, including word-finding difficulty and mental slowing. A basic metabolic panel is ordered.

Which lab abnormality is most likely to be found in this patient, and which of topiramate's mechanisms is responsible for it?

  1. Hyperammonemia due to inhibition of urea cycle enzymes.
  2. Hyponatremia due to syndrome of inappropriate antidiuretic hormone (SIADH).
  3. Decreased serum bicarbonate due to inhibition of carbonic anhydrase. (correct answer)
  4. Elevated creatine kinase due to drug-induced myopathy.
Explanation: Topiramate is a weak inhibitor of carbonic anhydrase. This action decreases the reabsorption of bicarbonate in the renal proximal tubule, which can lead to a non-anion gap hyperchloremic metabolic acidosis. While the cognitive side effects are a separate neurotoxic effect, the metabolic acidosis is a characteristic and monitorable adverse effect directly related to a specific mechanism of the drug. The other options are characteristic side effects of other antiepileptics (hyperammonemia with valproic acid, hyponatremia with oxcarbazepine).

Question 16

A physician in the emergency department administers an intravenous drug to a patient in status epilepticus. The drug acts by binding to an allosteric site on the GABA-A receptor, which increases the frequency of chloride channel opening in the presence of GABA.

Which antiepileptic drug was most likely administered?

  1. Phenobarbital
  2. Lorazepam (correct answer)
  3. Tiagabine
  4. Valproic acid
Explanation: This question tests the specific mechanistic distinction between benzodiazepines and barbiturates at the GABA-A receptor. Benzodiazepines, like lorazepam, increase the frequency of chloride channel opening. Barbiturates, like phenobarbital, increase the duration of channel opening. Tiagabine inhibits GABA reuptake, and valproic acid has multiple mechanisms, including increasing GABA synthesis and inhibiting its degradation, but neither primarily alters channel opening frequency. Lorazepam is a first-line agent for status epilepticus.

Question 17

A 6-month-old infant with infantile spasms has failed conventional therapies. Treatment with vigabatrin is being considered. The parents are counseled about a significant, irreversible toxicity associated with this drug.

What specific long-term monitoring is mandated for all patients receiving vigabatrin?

  1. Monthly complete blood counts for aplastic anemia.
  2. Serial echocardiograms for cardiac fibrosis.
  3. Annual renal ultrasounds for nephrocalcinosis.
  4. Regular ophthalmologic examinations for visual field defects. (correct answer)
Explanation: When you encounter questions about antiepileptic drug monitoring, focus on each medication's unique toxicity profile and required surveillance protocols. Vigabatrin carries a serious, dose-related risk of bilateral concentric visual field defects that can occur in up to 40% of patients. These defects are typically irreversible and asymptomatic until severe, making regular ophthalmologic monitoring absolutely essential. The visual field loss results from vigabatrin's toxicity to retinal cone cells, particularly in the peripheral retina. This is why answer D is correct – all patients on vigabatrin require baseline ophthalmologic examination before treatment and regular follow-up visual field testing every 6 months. A is incorrect because vigabatrin does not cause aplastic anemia or significant hematologic toxicity requiring routine blood count monitoring. B is wrong as vigabatrin is not associated with cardiac fibrosis – you might be thinking of medications like methysergide or ergot alkaloids. C is incorrect because nephrocalcinosis is not a recognized complication of vigabatrin therapy; this toxicity is more associated with medications like topiramate or carbonic anhydrase inhibitors. The visual field defects are particularly concerning because they're irreversible even after drug discontinuation, though they typically don't progress once the medication is stopped. This irreversible nature is why the parents in this case are being counseled about "significant, irreversible toxicity." Study tip: For antiepileptic drugs, memorize the major toxicity requiring monitoring for each: vigabatrin (visual fields), carbamazepine (blood counts), valproic acid (liver function), and phenytoin (gingival hyperplasia/osteomalacia).

Question 18

A patient is prescribed pregabalin for post-herpetic neuralgia. The patient asks how the medication works to relieve their pain.

Which of the following best describes the primary mechanism of action of pregabalin?

  1. It directly binds to and activates GABA-A and GABA-B receptors in the spinal cord.
  2. It inhibits the GABA-transaminase enzyme, leading to increased synaptic GABA levels.
  3. It blocks the reuptake of GABA from the synapse via the GAT-1 transporter.
  4. It binds to the alpha-2-delta subunit of presynaptic voltage-gated calcium channels. (correct answer)
Explanation: When encountering questions about gabapentinoids like pregabalin, remember that despite their name similarity to GABA, these drugs don't actually work through GABA mechanisms. This is a common source of confusion on pharmacology exams. Pregabalin works by binding specifically to the alpha-2-delta subunit of presynaptic voltage-gated calcium channels (VGCC). This binding reduces calcium influx into nerve terminals, which decreases the release of excitatory neurotransmitters like glutamate and substance P. By dampening this excitatory signaling, pregabalin effectively reduces neuropathic pain transmission, making it highly effective for conditions like post-herpetic neuralgia. Choice A is incorrect because pregabalin does not bind to or activate GABA receptors directly. While it may indirectly influence GABAergic tone, this isn't its primary mechanism. Choice B describes vigabatin's mechanism, which irreversibly inhibits GABA-transaminase to increase GABA levels. Choice C describes the mechanism of tiagabine, which blocks GABA reuptake via the GAT-1 transporter. The correct answer is D because pregabalin's therapeutic effect stems from its selective binding to the alpha-2-delta subunit of presynaptic VGCCs, reducing excitatory neurotransmitter release. Study tip: Remember that gabapentinoids (pregabalin and gabapentin) are "GABA imposters" – they're named for GABA but work through calcium channels, not GABA systems. Focus on calcium channel modulation when you see these drugs mentioned, especially for neuropathic pain conditions.

Question 19

A patient with epilepsy is found to have a serum sodium level of 126 mEq/L (normal 135-145 mEq/L) on routine lab work. The patient feels well otherwise. A review of their medication list is performed.

Initiation of which antiepileptic drug is the most likely cause of this patient's hyponatremia?

  1. Phenytoin
  2. Topiramate
  3. Levetiracetam
  4. Oxcarbazepine (correct answer)
Explanation: Oxcarbazepine, and to a lesser extent its predecessor carbamazepine, is frequently associated with hyponatremia, which can be clinically significant in up to 25% of patients. The mechanism is thought to involve an SIADH-like effect, increasing renal sensitivity to antidiuretic hormone. It is the most common electrolyte abnormality associated with this drug. The other agents listed are not typically associated with hyponatremia.

Question 20

A patient taking valproic acid for seizures presents to the emergency department with acute onset of lethargy, vomiting, and confusion. An urgent lab test reveals a serum ammonia level of 150 µmol/L (normal < 60 µmol/L).

In addition to discontinuing valproic acid, which intervention is most specifically indicated to manage this patient's condition?

  1. Administration of intravenous L-carnitine. (correct answer)
  2. Urgent hemodialysis to remove the drug.
  3. Empiric treatment with lactulose and rifaximin.
  4. Intravenous N-acetylcysteine infusion.
Explanation: Valproic acid can cause hyperammonemic encephalopathy by interfering with the urea cycle, partly through inducing a carnitine deficiency. L-carnitine is essential for the mitochondrial beta-oxidation of fatty acids, and its deficiency leads to a buildup of valproic acid metabolites that inhibit the urea cycle. Supplementation with L-carnitine can restore this function and facilitate ammonia clearance. Lactulose/rifaximin are used for hepatic encephalopathy, and N-acetylcysteine is the antidote for acetaminophen toxicity. Hemodialysis is a last resort for severe cases.