General Principles of Pharmacology

Correct Answer

C — Type A (augmented) reaction

Rationale

Type A adverse drug reactions are dose-dependent extensions of the drug's known pharmacological action. They are predictable from the drug's mechanism, common in clinical practice, and typically manageable by dose reduction. Warfarin bleeding is the prototype: it occurs because the drug is doing exactly what it is designed to do — reducing clotting factor synthesis — but to an excessive degree. Type B (idiosyncratic) reactions are unpredictable, unrelated to the drug's mechanism, and not dose-dependent. Hypersensitivity reactions are immune-mediated and occur in susceptible individuals regardless of dose.

Correct Answer

A — Immediate hypersensitivity reaction

Rationale

Immediate hypersensitivity reactions are immunoglobulin E-mediated and occur within minutes to one hour of drug exposure. Prior sensitization is required: an initial exposure generates drug-specific immunoglobulin E antibodies that bind to mast cells and basophils. On re-exposure, the drug cross-links these surface-bound antibodies, triggering degranulation and release of histamine and other mediators. Clinical manifestations range from urticaria to life-threatening anaphylaxis. Treatment requires epinephrine, and the drug must be permanently avoided. Delayed hypersensitivity reactions are cell-mediated and develop over hours to days. Type A reactions are dose-dependent extensions of drug mechanism. Type C reactions arise from prolonged use.

Correct Answer

D — Type B (idiosyncratic) reaction

Rationale

Type B adverse drug reactions are unpredictable, unrelated to the drug's known pharmacological mechanism, and occur only in susceptible individuals regardless of dose. Isoniazid hepatotoxicity is a classic example: it cannot be predicted from the drug's antituberculosis action, it affects only a small proportion of patients, and dose reduction does not prevent it. Type B reactions often reflect individual differences in drug metabolism or immune response. Because they are rare and unpredictable, they are difficult to detect in pre-approval clinical trials and represent a major focus of post-marketing pharmacovigilance. Type A reactions are dose-dependent and predictable from the drug's mechanism.

Correct Answer

B — Cytochrome P450 inducer

Rationale

Rifampin is one of the most potent cytochrome P450 inducers in clinical use — it upregulates multiple enzyme isoforms, particularly CYP3A4, CYP2C9, and CYP2C19. By increasing enzyme expression, rifampin accelerates the metabolism of co-administered drugs that use these pathways, reducing their plasma concentrations and risking therapeutic failure. Clinically consequential examples include reduced efficacy of oral contraceptives, antiretroviral drugs, warfarin, and immunosuppressants such as cyclosporine. The mnemonic grouping rifampin alongside phenytoin, carbamazepine, and phenobarbital helps identify the major cytochrome P450 inducers. Cytochrome P450 inhibitors reduce enzyme activity and raise drug levels, the opposite effect.

Correct Answer

A — Cytochrome P450 inhibitor

Rationale

Cytochrome P450 inhibitors reduce the metabolic activity of hepatic enzymes, slowing the breakdown of co-administered substrate drugs and raising their plasma concentrations. Fluconazole is a potent inhibitor of CYP2C9 and CYP3A4. When a patient on warfarin (a CYP2C9 substrate) is started on fluconazole, warfarin metabolism slows, its plasma concentration rises, and the international normalized ratio increases — sometimes dramatically — raising the risk of serious bleeding. This interaction is one of the most clinically important drug-drug interactions in outpatient practice. Cytochrome P450 inducers have the opposite effect — they increase enzyme activity and lower drug levels. A chelating agent forms insoluble complexes that impair drug absorption.

Correct Answer

C — Synergistic pharmacodynamic interaction

Rationale

A synergistic pharmacodynamic interaction occurs when the combined effect of two drugs exceeds the simple sum of their individual effects. Trimethoprim and sulfamethoxazole achieve this by blocking two consecutive steps in bacterial dihydrofolate synthesis — sulfamethoxazole blocks the earlier step and trimethoprim blocks the later conversion. Sequential blockade of the same pathway produces greater bactericidal activity than either drug alone at the same doses. This is exploited therapeutically to treat infections with lower doses of each component, reducing individual drug toxicity while enhancing efficacy. An additive interaction produces a combined effect equal to the sum of individual effects. An antagonistic interaction produces a combined effect less than the sum.

Correct Answer

B — Simvastatin plasma concentration increases because its metabolism is slowed by enzyme inhibition

Rationale

When a cytochrome P450 inhibitor blocks the enzyme that metabolizes a co-administered drug, the substrate drug is cleared more slowly than usual, and its plasma concentration rises above the expected level. For simvastatin, elevated plasma concentrations increase the risk of myopathy and rhabdomyolysis — both concentration-dependent adverse effects. Erythromycin is a CYP3A4 inhibitor, and simvastatin is a CYP3A4 substrate. This interaction is clinically important: prescribers must either avoid the combination, use an alternative antibiotic, or temporarily withhold the statin. Cytochrome P450 inhibition always raises substrate drug concentrations by slowing metabolism — it does not lower them.

Correct Answer

D — Oral contraceptive plasma concentrations decrease due to accelerated metabolism, risking contraceptive failure

Rationale

Cytochrome P450 inducers increase hepatic enzyme expression, accelerating the metabolism of co-administered substrate drugs and reducing their plasma concentrations. Carbamazepine induces CYP3A4 and other enzymes that metabolize ethinyl estradiol and progestins in combined oral contraceptives. The result is a reduction in contraceptive hormone levels sufficient to impair contraceptive efficacy, with reported cases of unintended pregnancy in women taking both drugs. This interaction extends to other antiepileptics including phenytoin, phenobarbital, and primidone, as well as rifampin. Women taking enzyme-inducing drugs must use alternative or additional contraception. Inducers lower drug levels — they do not raise them.

Correct Answer

A — Aluminum ions form insoluble chelate complexes with doxycycline in the gut, preventing its absorption

Rationale

Tetracyclines and fluoroquinolones form insoluble chelate complexes with polyvalent metal cations — aluminum, magnesium, calcium, and iron. When these cations are present in the gut at the same time as the antibiotic, the chelate complex is too insoluble to be absorbed, and the antibiotic passes through the gastrointestinal tract without entering the systemic circulation. The result is subtherapeutic drug levels and antibiotic failure. The solution is straightforward: separate the doses by at least two hours so the antibiotic is absorbed before the antacid reaches the gut, or use an antacid that does not contain polyvalent cations. Antacids do not inhibit cytochrome P450 enzymes or compete for plasma protein binding.

Correct Answer

C — Both drugs increase synaptic serotonin by different mechanisms, producing additive excess serotonergic activity

Rationale

Phenelzine is a monoamine oxidase inhibitor that blocks the enzymatic breakdown of serotonin, increasing its availability in the synapse. Tramadol inhibits serotonin reuptake, also increasing synaptic serotonin. When both mechanisms operate simultaneously, serotonin accumulates to levels that cause the serotonin syndrome: a clinical triad of neuromuscular excitability (clonus, hyperreflexia, tremor), autonomic instability (hyperthermia, tachycardia, diaphoresis), and altered mental status. This pharmacodynamic interaction does not involve changes in drug plasma concentrations — it arises from the additive effect of two serotonin-increasing mechanisms acting simultaneously. Monoamine oxidase inhibitors are contraindicated with all serotonergic drugs, and a washout period is required when switching between them.

Correct Answer

B — Dietary tyramine absorbed intact triggers massive norepinephrine release from sympathetic nerve terminals because its normal intestinal and hepatic metabolism by monoamine oxidase is blocked

Rationale

Normally, dietary tyramine — a vasoactive amine present in aged cheeses, cured meats, and fermented foods — is metabolized by monoamine oxidase in the gut wall and liver during absorption and first-pass transit, preventing it from reaching the systemic circulation. When monoamine oxidase is irreversibly inhibited by tranylcypromine, tyramine bypasses this presystemic metabolism and is absorbed intact. Tyramine then enters sympathetic nerve terminals, where it displaces norepinephrine from storage vesicles and causes massive norepinephrine release into the synapse. The resulting intense adrenergic stimulation produces severe hypertension that can cause intracerebral hemorrhage. Dietary counseling to avoid tyramine-rich foods is mandatory for every patient prescribed a monoamine oxidase inhibitor.

Correct Answer

D — The QT-prolonging effects of both drugs are additive at the cardiac ion channel level, producing cumulative repolarization impairment sufficient to trigger torsades de pointes

Rationale

QT prolongation results from impaired cardiac ventricular repolarization, most often through blockade of cardiac potassium channels. When two drugs each producing modest QT prolongation are combined, their effects on repolarization are additive — the cumulative QT prolongation can exceed the threshold for torsades de pointes, a potentially fatal polymorphic ventricular tachycardia. Neither drug alone at its usual dose may produce dangerous QT prolongation, but the combination can. This additive pharmacodynamic interaction occurs without any change in plasma concentrations of either drug. Before adding any QT-prolonging drug to a patient already taking one, a baseline electrocardiogram and assessment of other risk factors — hypokalemia, bradycardia, female sex, underlying cardiac disease — are warranted.

Correct Answer

A — Beta-2 receptor blockade removes bronchodilatory tone from endogenous catecholamines, precipitating bronchoconstriction in airways already prone to hyperreactivity

Rationale

Propranolol is a non-selective beta-adrenergic blocker that blocks both beta-1 receptors in the heart and beta-2 receptors in bronchial smooth muscle. Endogenous epinephrine maintains baseline bronchodilatory tone through beta-2 receptor activation. When propranolol blocks beta-2 receptors, this bronchodilatory influence is lost. In a patient with asthma whose airways are hyperreactive, the removal of beta-2-mediated bronchodilation can precipitate life-threatening bronchoconstriction. This is a pharmacodynamic drug-disease interaction: the drug's mechanism directly worsens the patient's disease state. Cardioselective beta-1 blockers carry lower but not absent risk in asthma, and even these should be used with caution.

Correct Answer

C — Prostaglandin inhibition causes sodium and water retention and reduces renal perfusion pressure, worsening fluid overload and impairing kidney function

Rationale

In patients with heart failure, reduced cardiac output activates compensatory mechanisms that rely on prostaglandins to maintain adequate renal perfusion. Non-steroidal anti-inflammatory drugs inhibit cyclooxygenase, reducing prostaglandin synthesis. This loss of prostaglandin-mediated renal afferent arteriolar dilation reduces glomerular filtration rate, impairing kidney function. Simultaneously, prostaglandin inhibition promotes sodium and water retention, worsening the fluid overload that defines decompensated heart failure. This is a direct pharmacodynamic drug-disease interaction: the mechanism of the drug — cyclooxygenase inhibition — is inherently harmful in the pathological context of heart failure. Non-steroidal anti-inflammatory drugs also blunt the effectiveness of diuretics and angiotensin-converting enzyme inhibitors used to treat heart failure.

Correct Answer

D — Fluconazole inhibits the cytochrome P450 enzyme that metabolizes warfarin, raising warfarin plasma concentration and anticoagulant effect

Rationale

Warfarin is metabolized primarily by CYP2C9. Fluconazole is a potent inhibitor of CYP2C9. When fluconazole is added, warfarin metabolism slows, its plasma concentration rises, and the anticoagulant effect intensifies — reflected in the rising international normalized ratio. This is one of the most clinically consequential drug-drug interactions in outpatient medicine, because the combination is common — both drugs are widely prescribed — and the consequences of unrecognized supratherapeutic anticoagulation can be fatal. When fluconazole must be used in a patient on warfarin, warfarin dose reduction and close international normalized ratio monitoring are required. Cytochrome P450 induction would accelerate warfarin metabolism and reduce its effect, the opposite of what occurred here.

Correct Answer

A — Phenelzine prevents serotonin breakdown while tramadol inhibits serotonin reuptake, producing additive excess synaptic serotonin and serotonin syndrome

Rationale

Phenelzine irreversibly inhibits monoamine oxidase, the enzyme responsible for degrading synaptic serotonin. Tramadol, in addition to its opioid receptor activity, inhibits the serotonin reuptake transporter, increasing synaptic serotonin. When both mechanisms operate simultaneously, synaptic serotonin accumulates to levels that overwhelm normal regulatory capacity, producing serotonin syndrome. The clinical triad is neuromuscular excitability (clonus, hyperreflexia, tremor), autonomic instability (hyperthermia, tachycardia, diaphoresis), and altered mental status. This pharmacodynamic interaction does not involve changes in plasma drug concentrations — it is a direct consequence of additive serotonergic excess. Monoamine oxidase inhibitors must not be combined with any drug that increases synaptic serotonin by any mechanism.

Correct Answer

C — Heparin, because its large molecular size prevents placental transfer, avoiding fetal exposure to the drug

Rationale

Warfarin is a small lipophilic molecule that crosses the placenta readily and causes warfarin embryopathy when used in the first trimester, as well as fetal bleeding in later pregnancy — it is contraindicated throughout pregnancy. Heparin, including low-molecular-weight heparins such as enoxaparin, is a large polyanion that cannot cross the placental barrier. The fetus is therefore not exposed to the drug, and heparin anticoagulation does not cause fetal harm. Heparin requires subcutaneous or intravenous administration because it has no oral bioavailability, but this is preferable to the teratogenic risk of warfarin. Direct oral anticoagulants including rivaroxaban are contraindicated in pregnancy due to fetal risk, and aspirin and clopidogrel do not provide sufficient anticoagulation for a mechanical valve.

Correct Answer

B — Rifampin induces cytochrome P450 enzymes, accelerating the metabolism of contraceptive hormones and reducing their plasma concentrations below effective levels

Rationale

Rifampin is one of the most potent cytochrome P450 inducers in clinical use. By upregulating CYP3A4 and other metabolic enzymes, rifampin dramatically accelerates the hepatic metabolism of ethinyl estradiol and progestins, lowering their plasma concentrations to levels insufficient to suppress ovulation reliably. The enzyme induction develops over the first one to two weeks of rifampin treatment and persists for approximately four weeks after the drug is stopped — explaining why additional contraception is needed for one month after completing the rifampin course. This interaction is well-documented and has resulted in unintended pregnancies. Rifampin is an inducer, not an inhibitor, so it lowers rather than raises drug concentrations.