Serotonin Pharmacology

Correct Answer

C — Fluoxetine

Rationale

Fluoxetine has the longest half-life of any selective serotonin reuptake inhibitor, due in part to its active metabolite norfluoxetine which persists for weeks after the parent drug is discontinued. This distinguishes fluoxetine from sertraline and escitalopram, which have intermediate half-lives, and from paroxetine, which has the shortest half-life of the major selective serotonin reuptake inhibitors and carries the highest risk of discontinuation syndrome.

Correct Answer

A — Paroxetine

Rationale

Paroxetine is the most anticholinergic of the selective serotonin reuptake inhibitors, a property that contributes to adverse effects including dry mouth, constipation, and urinary retention. It also carries the highest discontinuation syndrome risk of the class due to its short half-life. Fluoxetine, sertraline, and escitalopram have much less anticholinergic activity and are better tolerated in patients sensitive to these effects.

Correct Answer

B — Duloxetine

Rationale

Duloxetine is classified as a serotonin-norepinephrine reuptake inhibitor, blocking both the serotonin transporter and the norepinephrine transporter across its full therapeutic dose range. Mirtazapine is classified as an alpha-2 adrenergic antagonist and serotonin type 2/3 blocker. Bupropion is classified as a dopamine and norepinephrine reuptake inhibitor with no serotonergic activity. Trazodone is classified as a serotonin type 2 antagonist with weak serotonin transporter inhibition.

Correct Answer

D — Non-selective irreversible monoamine oxidase inhibitor

Rationale

Phenelzine is classified as a non-selective irreversible monoamine oxidase inhibitor — it inhibits both monoamine oxidase A and monoamine oxidase B permanently, requiring new enzyme synthesis before activity recovers. A selective reversible monoamine oxidase A inhibitor describes moclobemide. Selective monoamine oxidase B inhibition at low doses describes selegiline, used in Parkinson's disease. Serotonin-norepinephrine reuptake inhibition describes duloxetine and venlafaxine.

Correct Answer

B — Alpha-2 adrenergic antagonist and serotonin type 2 and type 3 receptor blocker

Rationale

Mirtazapine is classified as an alpha-2 adrenergic receptor antagonist combined with serotonin type 2 and type 3 receptor blockade. This dual classification distinguishes it from other antidepressants — it increases norepinephrine and serotonin release by removing alpha-2 autoreceptor inhibition, while simultaneously blocking downstream serotonin type 2 and type 3 receptors. Selective serotonin reuptake inhibitor describes fluoxetine and related agents. Serotonin type 2 antagonism with weak serotonin transporter inhibition describes trazodone. Dopamine and norepinephrine reuptake inhibition describes bupropion.

Correct Answer

A — Dopamine and norepinephrine reuptake inhibitor

Rationale

Bupropion is classified as a dopamine and norepinephrine reuptake inhibitor with no serotonergic activity — a classification that distinguishes it from every other antidepressant in common use. It does not inhibit the serotonin transporter and does not carry serotonin syndrome risk. Selective serotonin reuptake inhibition describes fluoxetine and related agents. Serotonin-norepinephrine reuptake inhibition describes duloxetine and venlafaxine. Serotonin type 2 antagonism describes trazodone.

Correct Answer

D — Fluoxetine's active metabolite norfluoxetine has a prolonged half-life that continues to inhibit serotonin reuptake for weeks after fluoxetine is stopped

Rationale

Norfluoxetine, the active metabolite of fluoxetine, has a half-life of 7 to 15 days. Even after fluoxetine itself clears the body, norfluoxetine continues to block the serotonin transporter for weeks, maintaining the risk of serotonin syndrome if a monoamine oxidase inhibitor is started too soon. A 5-week washout is required to ensure norfluoxetine has cleared. While fluoxetine does inhibit cytochrome P450 2D6, this does not explain the washout duration requirement. Fluoxetine inhibits the serotonin transporter reversibly, not irreversibly. Receptor affinity displacement is not the mechanism governing washout timing.

Correct Answer

A — Hyponatremia caused by serotonin-mediated stimulation of antidiuretic hormone release

Rationale

Selective serotonin reuptake inhibitors can cause the syndrome of inappropriate antidiuretic hormone secretion, producing hyponatremia. The mechanism involves increased serotonergic activity stimulating antidiuretic hormone release from the hypothalamus, leading to water retention and dilutional hyponatremia. This effect is most pronounced in elderly patients, in whom it can manifest as confusion — a presentation that may be mistaken for other causes of altered mental status. The other electrolyte abnormalities listed are not recognized adverse effects of selective serotonin reuptake inhibitors.

Correct Answer

C — Higher doses produce increasing norepinephrine transporter blockade, raising norepinephrine levels and blood pressure

Rationale

Venlafaxine exhibits dose-dependent pharmacology: at low doses its effects are predominantly serotonergic, resembling a selective serotonin reuptake inhibitor. As the dose increases, norepinephrine transporter blockade becomes progressively more prominent, raising synaptic norepinephrine levels and increasing peripheral vascular tone — which can elevate blood pressure. This property distinguishes venlafaxine from duloxetine, which provides balanced serotonin transporter and norepinephrine transporter inhibition at all doses. Venlafaxine does not inhibit monoamine oxidase A. QT prolongation is associated with other agents such as citalopram and ondansetron, not venlafaxine.

Correct Answer

B — Norepinephrine transporter blockade enhances descending noradrenergic inhibition of pain transmission in the spinal cord

Rationale

The descending noradrenergic pain control system — originating in brainstem nuclei and projecting to the dorsal horn of the spinal cord — normally suppresses pain signal transmission. By blocking the norepinephrine transporter, duloxetine increases synaptic norepinephrine in these pathways, enhancing their inhibitory effect on ascending pain signals. This is the mechanism underlying its approved indications for diabetic peripheral neuropathy, fibromyalgia, and chronic musculoskeletal pain. Serotonin type 3 receptor blockade mediates antiemetic effects, not analgesia. Serotonin transporter blockade does not reduce prostaglandin synthesis. Duloxetine does not inhibit monoamine oxidase A.

Correct Answer

D — Meperidine inhibits serotonin reuptake, and combined with monoamine oxidase inhibitor-mediated blockade of serotonin degradation, causes serotonin accumulation to toxic levels

Rationale

Meperidine possesses serotonin reuptake inhibitor activity in addition to its opioid properties. When combined with a monoamine oxidase inhibitor, which blocks serotonin degradation, the result is dangerous accumulation of serotonin — producing serotonin syndrome with clonus, hyperthermia, and autonomic instability. Morphine is the safe opioid alternative because it lacks this serotonergic activity. Meperidine is not a substrate for monoamine oxidase A metabolism. Meperidine does not stimulate serotonin release or activate serotonin type 3 receptors.

Correct Answer

A — Inhibition of monoamine oxidase A in the gut allows dietary tyramine to be absorbed intact, where it displaces norepinephrine from adrenergic nerve terminals, producing a hypertensive surge

Rationale

Normally, monoamine oxidase A in the gut wall and liver inactivates dietary tyramine before it reaches systemic circulation, preventing any sympathomimetic effect. When monoamine oxidase A is inhibited by drugs such as tranylcypromine or phenelzine, tyramine is absorbed intact into the systemic circulation, enters adrenergic nerve terminals, and acts as an indirect sympathomimetic by displacing stored norepinephrine — producing the massive blood pressure surge known as the tyramine or cheese reaction. Tyramine does not directly activate serotonin type 2A receptors or inhibit the serotonin transporter. The reaction is noradrenergic, not serotonergic.

Correct Answer

C — Bupropion lowers the seizure threshold, and patients with eating disorders are already at elevated seizure risk from electrolyte disturbances

Rationale

Bupropion lowers the seizure threshold in a dose-dependent fashion. Patients with anorexia nervosa or bulimia nervosa are at elevated baseline seizure risk because purging behaviors — vomiting and laxative use — cause hypokalemia, hypomagnesemia, and other electrolyte disturbances that reduce the seizure threshold independently. Adding bupropion in this context further increases seizure risk to unacceptable levels. Bupropion has no serotonergic activity. While bupropion may suppress appetite, this is not the basis for the eating disorder contraindication. The contraindication is explicitly about seizure risk.

Correct Answer

B — Alpha-1 adrenergic receptor blockade impairs the sympathetically mediated detumescence that normally terminates erection

Rationale

Normal detumescence — the return of the penis to a flaccid state after erection — depends on alpha-1 adrenergic receptor activation in penile smooth muscle, which causes contraction and reduces blood inflow. Trazodone blocks alpha-1 adrenergic receptors, impairing this sympathetic mechanism and allowing engorgement to persist. The result can be priapism, which requires urgent medical attention as prolonged ischemia can cause permanent erectile dysfunction. Serotonin type 2 antagonism is trazodone's primary receptor mechanism but does not account for priapism. Serotonin transporter inhibition by trazodone is weak and is not the mechanism. Histamine H1 blockade accounts for sedation, not priapism.

Correct Answer

C — Fluoxetine's active metabolite norfluoxetine has a prolonged half-life and continues to inhibit serotonin reuptake long after fluoxetine itself has cleared

Rationale

Norfluoxetine, the pharmacologically active metabolite of fluoxetine, has a half-life of 7 to 15 days. At two weeks after stopping fluoxetine, substantial norfluoxetine concentrations remain, continuing to block the serotonin transporter. When phenelzine is added — blocking serotonin degradation — the combination of ongoing reuptake inhibition and impaired degradation produces toxic serotonin accumulation. A 5-week washout is required after fluoxetine specifically because of norfluoxetine's persistence; other selective serotonin reuptake inhibitors with shorter half-lives require only 14 days. Fluoxetine does not irreversibly inhibit monoamine oxidase A. Receptor upregulation does not explain the washout requirement. While fluoxetine does inhibit cytochrome P450 2D6, phenelzine toxicity from this mechanism is not the established explanation for the washout rule.

Correct Answer

D — Bupropion

Rationale

Bupropion is a dopamine and norepinephrine reuptake inhibitor with no serotonergic activity. Because sexual dysfunction caused by antidepressants is mediated through serotonin signaling — specifically serotonin transporter blockade raising serotonin at receptors that inhibit sexual response — bupropion does not produce this adverse effect. It is the preferred alternative for patients who cannot tolerate the sexual side effects of selective serotonin reuptake inhibitors or serotonin-norepinephrine reuptake inhibitors. Paroxetine would worsen sexual dysfunction — it is the most anticholinergic and has the highest discontinuation syndrome risk of the selective serotonin reuptake inhibitors. Venlafaxine and duloxetine are serotonin-norepinephrine reuptake inhibitors that also inhibit the serotonin transporter and carry the same sexual dysfunction risk.

Correct Answer

A — Mirtazapine

Rationale

Mirtazapine's blockade of histamine H1 receptors produces pronounced sedation and appetite stimulation — properties that are adverse effects in many patients but become therapeutic advantages in patients with depression-associated insomnia and significant weight loss. Its alpha-2 adrenergic antagonism increases norepinephrine and serotonin release, providing antidepressant efficacy without serotonin transporter blockade and therefore without the sexual dysfunction common to selective serotonin reuptake inhibitors. Fluoxetine and venlafaxine do not cause sedation or appetite stimulation and may worsen insomnia. Bupropion is activating and associated with weight loss — the opposite of what is needed here.

Correct Answer

B — Increased serotonergic activity stimulates antidiuretic hormone release, causing water retention and dilutional hyponatremia

Rationale

Selective serotonin reuptake inhibitors can cause the syndrome of inappropriate antidiuretic hormone secretion through a serotonin-mediated mechanism: increased synaptic serotonin stimulates antidiuretic hormone release from the hypothalamus, leading to inappropriate water retention and dilutional hyponatremia. This effect is most pronounced in elderly patients, in whom it can present as confusion — a classic Step 1 presentation. The effect occurs with escitalopram, fluoxetine, sertraline, and all other selective serotonin reuptake inhibitors. Escitalopram does not inhibit aldosterone secretion, block renal sodium channels, or have meaningful anticholinergic activity — the latter property is absent from escitalopram compared to paroxetine.