Antidepressant Drugs

Correct Answer

C — Nortriptyline

Rationale

Nortriptyline is classified as a secondary amine tricyclic antidepressant. The secondary amines — nortriptyline and desipramine — have relatively less off-target receptor binding and are better tolerated than the tertiary amines, particularly in elderly patients. Amitriptyline, imipramine, and clomipramine are all classified as tertiary amine tricyclic antidepressants. Nortriptyline is actually the active metabolite of amitriptyline, produced by demethylation of the tertiary amine parent compound.

Correct Answer

A — Clomipramine

Rationale

Clomipramine is classified as the tricyclic antidepressant with the most potent serotonin reuptake inhibition in its class. This pharmacological property distinguishes it from amitriptyline, nortriptyline, and desipramine, which have more balanced or norepinephrine-predominant reuptake inhibition profiles. Clomipramine's potent serotonin reuptake inhibition underlies its classification as the most efficacious pharmacological agent for obsessive-compulsive disorder, reserved for cases that have not responded to selective serotonin reuptake inhibitor treatment. Amitriptyline and nortriptyline are classified by their use in neuropathic pain and migraine prophylaxis. Desipramine is a secondary amine with predominantly norepinephrine reuptake inhibition.

Correct Answer

B — Phenelzine

Rationale

Phenelzine is classified as an irreversible monoamine oxidase inhibitor — it forms a permanent covalent bond with the monoamine oxidase enzyme, inactivating it for the lifetime of that enzyme molecule. Tranylcypromine and isocarboxazid share this irreversible classification. Moclobemide is classified as a reversible inhibitor of monoamine oxidase A; it binds non-covalently and its inhibitory effect dissipates within hours of stopping, which substantially reduces its dietary interaction risk and requires only a 24-hour washout before starting serotonergic agents. Selegiline at the low oral doses used in Parkinson's disease selectively inhibits monoamine oxidase B in a mechanism-dependent but irreversible fashion — however, its selectivity for monoamine oxidase B rather than monoamine oxidase A at these doses is the pharmacologically relevant distinguishing feature, not its reversibility status.

Correct Answer

D — Doxepin

Rationale

Doxepin is the only tricyclic antidepressant with a Food and Drug Administration approval specifically for insomnia, granted for low-dose formulations of 3 mg and 6 mg. At these doses, doxepin's potent histamine H1 receptor blockade produces sedation without the full anticholinergic burden of antidepressant doses, making it a distinct pharmacological option for sleep maintenance insomnia. Amitriptyline, nortriptyline, and imipramine are used off-label for insomnia in clinical practice but do not hold Food and Drug Administration approvals for this specific indication. Imipramine is approved for nocturnal enuresis in children, not for insomnia in adults.

Correct Answer

A — Moclobemide

Rationale

Moclobemide is classified as a reversible inhibitor of monoamine oxidase A — it binds non-covalently and its inhibitory effect dissipates within hours of stopping the drug. This reversibility substantially reduces the tyramine dietary interaction risk compared to the classical irreversible agents, because sufficient monoamine oxidase A activity can recover quickly to handle dietary tyramine loads. Moclobemide is used in Europe, Canada, and Australia but is not Food and Drug Administration-approved in the United States. Its washout period before starting serotonergic agents is only 24 hours, compared to the two weeks required after stopping irreversible monoamine oxidase inhibitors. Phenelzine, tranylcypromine, and isocarboxazid are all classified as irreversible monoamine oxidase inhibitors.

Correct Answer

C — Amitriptyline

Rationale

Amitriptyline is classified as a tertiary amine tricyclic antidepressant. The tertiary amines — amitriptyline, imipramine, clomipramine, and doxepin — have stronger serotonin reuptake inhibition and substantially more anticholinergic, antihistaminic, and alpha-1 antagonist activity than the secondary amines. They are harder to tolerate and more sedating, and are listed on the Beers Criteria as inappropriate for routine use in elderly patients. Nortriptyline and desipramine are classified as secondary amines and have relatively less off-target receptor binding. Amitriptyline is demethylated in the body to nortriptyline, its secondary amine active metabolite.

Correct Answer

D — Alpha-1 adrenergic receptor blockade

Rationale

Alpha-1 adrenergic receptors on peripheral blood vessels mediate the reflex vasoconstriction that normally compensates for the drop in blood pressure that occurs when a person stands. Tricyclic antidepressants block these receptors, impairing this compensatory response and causing blood pressure to fall on standing — orthostatic hypotension. In elderly patients this produces syncope, falls, and consequent hip fractures and head injuries. Nortriptyline is preferred when a tricyclic antidepressant is clinically required because it has less alpha-1 antagonism than the tertiary amines. Histamine H1 blockade produces sedation and weight gain. Muscarinic receptor blockade produces the anticholinergic syndrome: dry mouth, constipation, urinary retention, and confusion. Serotonin-2A blockade is not a primary tricyclic antidepressant mechanism linked to orthostatic hypotension.

Correct Answer

B — Alkalinization of the blood reduces the drug's affinity for the sodium channel, and the raised serum sodium increases the electrochemical driving force for sodium entry

Rationale

Sodium bicarbonate treats tricyclic antidepressant cardiac toxicity through two complementary mechanisms. First, alkalinizing the blood to a target pH of approximately 7.45 to 7.55 reduces the tricyclic antidepressant molecule's affinity for the cardiac sodium channel, decreasing the degree of channel blockade. Second, the sodium load delivered by sodium bicarbonate raises serum sodium concentration, increasing the electrochemical driving force for sodium ions to enter the channel — partially overcoming the blockade through a mass-action effect. The clinical goal is narrowing of the QRS complex toward baseline. Sodium bicarbonate does not competitively displace drug from the channel pore, does not enhance renal elimination of tricyclic antidepressants (which have a very large volume of distribution making renal clearance negligible), and does not act through potassium mechanisms.

Correct Answer

A — Phenytoin also blocks cardiac sodium channels, which would worsen the sodium channel blockade already caused by the tricyclic antidepressant and increase arrhythmia risk

Rationale

Phenytoin is an antiepileptic drug that controls seizures partly through cardiac and neuronal sodium channel blockade. In tricyclic antidepressant overdose, where life-threatening QRS widening and ventricular arrhythmia are already produced by tricyclic antidepressant sodium channel blockade, adding phenytoin would compound this blockade and risk precipitating fatal arrhythmia. Benzodiazepines are the correct first-line treatment for tricyclic antidepressant overdose-associated seizures — they control seizures through gamma-aminobutyric acid receptor potentiation without sodium channel effects, making them safe in this setting. Phenytoin's mechanism of harm in this context is pharmacodynamic, not pharmacokinetic.

Correct Answer

C — Monoamine oxidase A, found in noradrenergic and serotonergic neurons, the intestinal mucosa, and the liver

Rationale

Monoamine oxidase A preferentially degrades serotonin and norepinephrine and is distributed in noradrenergic and serotonergic neurons, the intestinal mucosa, and the liver. Its presence in the intestinal mucosa and liver is what makes it the critical enzyme for tyramine first-pass metabolism — normally, tyramine ingested from food is almost completely degraded by monoamine oxidase A during absorption, preventing it from reaching systemic circulation. Monoamine oxidase A inhibition is also necessary for antidepressant effect, since serotonin and norepinephrine are the relevant substrates. Monoamine oxidase B preferentially degrades dopamine and phenylethylamine and is found in dopaminergic neurons, platelets, and glial cells — its selective inhibition at low doses, as with selegiline in Parkinson's disease, does not require dietary tyramine restriction because gut monoamine oxidase A remains functional.

Correct Answer

D — Tyramine enters adrenergic nerve terminals via the norepinephrine reuptake transporter and displaces norepinephrine from vesicular storage, causing a massive surge of norepinephrine release

Rationale

Tyramine is an indirect sympathomimetic. Normally, tyramine ingested from food is almost completely degraded by monoamine oxidase A in the intestinal lining and liver during first-pass metabolism and never reaches systemic circulation in meaningful amounts. When monoamine oxidase A is irreversibly inhibited, this protective extraction fails and dietary tyramine enters systemic circulation intact. Tyramine then enters adrenergic nerve terminals by using the norepinephrine reuptake transporter — the same transporter that normally reuptakes norepinephrine from the synapse — and once inside, it displaces norepinephrine from its vesicular storage sites in massive quantities. The resulting surge of norepinephrine release causes the sudden severe hypertension, pounding headache, diaphoresis, and flushing of the tyramine pressor response. Tyramine does not directly stimulate adrenergic receptors, does not inhibit monoamine oxidase A in the adrenal medulla, and does not block the norepinephrine reuptake transporter.

Correct Answer

B — Mood reactivity, hypersomnia, increased appetite with carbohydrate craving, leaden paralysis, and rejection sensitivity

Rationale

The features in option B describe atypical depression, the depression subtype for which monoamine oxidase inhibitors are the most effective pharmacological treatment. Mood reactivity — the hallmark feature — means the patient's mood brightens in response to positive events, distinguishing atypical depression from the mood that is fixed and unresponsive to circumstances in melancholic depression. Accompanying features include hypersomnia, increased appetite with carbohydrate craving, leaden paralysis (a heavy, weighted feeling in the arms and legs), and rejection sensitivity. Randomized controlled trials have demonstrated monoamine oxidase inhibitor superiority over tricyclic antidepressants specifically in atypical depression. The features in option A describe melancholic depression, which responds better to tricyclic antidepressants and selective serotonin reuptake inhibitors. Option C describes classic melancholia. Option D describes psychotic depression, which requires antipsychotic augmentation.

Correct Answer

A — Anticholinergic central nervous system effects — confusion, falls, and cognitive impairment — are particularly dangerous in elderly patients

Rationale

Tricyclic antidepressants are listed on the Beers Criteria because their anticholinergic effects on the central nervous system — confusion, delirium, cognitive impairment, and sedation — are particularly harmful in elderly patients, who have reduced central nervous system reserve and are more vulnerable to anticholinergic toxicity than younger adults. These effects increase fall risk, cause or worsen cognitive impairment, and can precipitate delirium. The alpha-1 adrenergic blockade producing orthostatic hypotension further compounds fall risk in this population. When a tricyclic antidepressant is clinically required in an older patient, nortriptyline is the preferred choice because it has the least anticholinergic and alpha-1 antagonist activity of the commonly used agents. Reduced renal clearance and cytochrome P450 interactions are pharmacokinetic considerations; the Beers Criteria classification rests on pharmacodynamic concerns — specifically, anticholinergic and alpha-1 effects that are more dangerous in elderly patients.

Correct Answer

C — Recovery of monoamine oxidase activity requires synthesis of new enzyme protein, a process that takes approximately two weeks regardless of drug plasma levels

Rationale

Irreversible monoamine oxidase inhibitors form permanent covalent bonds with the monoamine oxidase enzyme, permanently inactivating each enzyme molecule they bind. Once the drug itself is eliminated from plasma — which occurs within days — the inactivated enzyme molecules remain non-functional. Recovery of monoamine oxidase activity depends entirely on the body synthesizing new enzyme protein to replace the inactivated molecules, a process that takes approximately two weeks. This is why the drug's pharmacological effect — including the dietary tyramine interaction and the risk of serotonin syndrome with serotonergic combinations — persists long after plasma drug levels fall to zero. The washout requirement is pharmacodynamic, not pharmacokinetic. Monoamine oxidase inhibitors do not accumulate in fat tissue, do not have relevant active metabolites, and do not bind serotonin receptors.

Correct Answer

C — Blood alkalinization reduces amitriptyline's affinity for the cardiac sodium channel, and the sodium load helps overcome the channel blockade

Rationale

Amitriptyline overdose produces QRS widening and hemodynamic instability through blockade of fast cardiac sodium channels. Sodium bicarbonate treats this toxicity through two complementary mechanisms. First, alkalinizing the blood to a target pH of approximately 7.45 to 7.55 reduces the tricyclic antidepressant molecule's binding affinity for the sodium channel, decreasing the degree of blockade. Second, the sodium load delivered by sodium bicarbonate raises serum sodium concentration and increases the electrochemical driving force for sodium ions to enter through the channel — a mass-action effect that partially overcomes the blockade. QRS duration above 100 milliseconds warrants sodium bicarbonate treatment without waiting for further deterioration. Tricyclic antidepressants have a very large volume of distribution, making urinary alkalinization and enhanced renal elimination clinically ineffective. Sodium bicarbonate does not chelate the drug and does not act through potassium mechanisms.

Correct Answer

A — With monoamine oxidase A inhibited, dietary tyramine bypassed gut degradation, entered adrenergic nerve terminals via the norepinephrine reuptake transporter, and displaced norepinephrine from vesicles in massive quantities

Rationale

Phenelzine is an irreversible monoamine oxidase A inhibitor. Normally, tyramine in aged foods is almost completely degraded by monoamine oxidase A in the gut and liver during first-pass metabolism and never reaches systemic circulation in meaningful amounts. With monoamine oxidase A irreversibly inhibited, this protective extraction fails entirely and dietary tyramine enters systemic circulation intact. Tyramine then acts as an indirect sympathomimetic — entering adrenergic nerve terminals through the norepinephrine reuptake transporter and displacing norepinephrine from vesicular storage in massive quantities. The resulting surge of norepinephrine causes the sudden severe hypertension, pounding headache, diaphoresis, and flushing seen here. Aged cheeses carry the highest tyramine content of any food category and are among the most dangerous foods for patients on irreversible monoamine oxidase inhibitors. Tyramine does not directly stimulate adrenergic receptors and does not affect phenelzine metabolism.

Correct Answer

D — Monoamine oxidase inhibitors

Rationale

This patient's clinical features — mood reactivity (mood brightens in response to positive events), hypersomnia, carbohydrate craving, and leaden paralysis — define atypical depression, the depression subtype for which monoamine oxidase inhibitors are the most effective pharmacological treatment. Randomized controlled trials have demonstrated monoamine oxidase inhibitor superiority over tricyclic antidepressants specifically in atypical depression, making them the evidence-based choice after selective serotonin reuptake inhibitor failure in this subtype. Mood reactivity is the essential distinguishing feature — patients with melancholic depression have a persistently low mood that does not brighten with positive events. Serotonin-norepinephrine reuptake inhibitors and norepinephrine-dopamine reuptake inhibitors are effective antidepressants but do not have the controlled trial evidence base in atypical depression that monoamine oxidase inhibitors hold. Tricyclic antidepressants are less effective than monoamine oxidase inhibitors for atypical depression specifically.

Correct Answer

B — Amitriptyline's alpha-1 adrenergic receptor blockade impairs the reflex vasoconstriction that normally compensates for the blood pressure drop on standing

Rationale

When a person stands, gravitational pooling of blood in the lower extremities causes a transient drop in venous return and blood pressure. Normally, alpha-1 adrenergic receptors on peripheral blood vessels mediate a rapid reflex vasoconstriction that compensates for this drop and restores blood pressure within seconds. Amitriptyline blocks these alpha-1 receptors, impairing this compensatory response and allowing blood pressure to remain low on standing — producing orthostatic hypotension. In elderly patients this is particularly dangerous because reduced barovascular reserve increases fall risk, with consequent hip fractures and head injuries. Nortriptyline is the preferred tricyclic antidepressant in elderly patients when one is clinically required because it has the least alpha-1 antagonism of the commonly used agents. Histamine H1 blockade produces sedation, not selective postural hypotension. Muscarinic blockade increases heart rate through vagal inhibition. Sodium channel blockade at therapeutic doses does not produce clinically meaningful cardiac output reduction.