Antifungal Agents

Correct Answer

B — Amphotericin B and nystatin

Rationale

Amphotericin B and nystatin are both polyene antifungals, a class defined by a large macrolide ring structure with a polyene chain. Both bind ergosterol in the fungal cell membrane and disrupt membrane integrity. Fluconazole and itraconazole are triazole antifungals. Caspofungin and micafungin are echinocandins. Terbinafine and griseofulvin are agents that act on the ergosterol biosynthesis pathway and fungal mitosis, respectively, and belong to neither the polyene nor the azole class.

Correct Answer

A — Liposomal amphotericin B

Rationale

Liposomal amphotericin B consists of small unilamellar liposomes with the drug intercalated into the phospholipid bilayer membrane of the liposome. Amphotericin B lipid complex consists of ribbon-like lipid bilayer structures, not closed liposomes. Amphotericin B colloidal dispersion consists of disk-shaped cholesteryl sulfate complexes. Amphotericin B deoxycholate is the conventional formulation using sodium deoxycholate as a micellar solubilizing vehicle and contains no lipid encapsulation structure.

Correct Answer

B — Broad spectrum — active against most pathogenic fungi, including yeasts, molds, and dimorphic fungi

Rationale

Amphotericin B is classified as a broad-spectrum antifungal agent. Its activity encompasses most clinically important pathogenic fungi, including Candida species, Aspergillus species, Cryptococcus neoformans, the endemic dimorphic fungi (Histoplasma, Coccidioides, Blastomyces), and the agents of mucormycosis. This breadth of coverage is why amphotericin B remains an important agent for empiric therapy of serious fungal infections when the causative organism is not yet known. Amphotericin B has no activity against bacteria — its mechanism of action requires ergosterol in the target membrane, which bacteria do not possess. Dermatophyte coverage alone would describe a narrow-spectrum agent such as terbinafine or griseofulvin.

Correct Answer

C — Topical and oral luminal formulations only — no systemic or intravenous preparations are available

Rationale

Nystatin is classified as a topical and luminal antifungal agent. It is formulated as a cream, ointment, powder, and oral suspension. The oral suspension acts locally in the mouth and gastrointestinal tract and is not absorbed systemically. No intravenous formulation of nystatin is available for clinical use because the drug causes severe toxicity at concentrations required for systemic antifungal activity. This is the fundamental distinction between nystatin and amphotericin B within the polyene class: amphotericin B can be administered intravenously for systemic infections, while nystatin cannot. Nystatin is therefore confined to treatment of mucosal and superficial candidal infections.

Correct Answer

D — Fungicidal — kills fungi directly at standard clinical concentrations

Rationale

Amphotericin B is classified as a fungicidal agent at standard clinical concentrations. By forming transmembrane pores in the fungal cell membrane, amphotericin B causes rapid ion efflux, collapse of membrane potential, and failure of essential active transport — changes that kill the organism directly rather than merely inhibiting its growth. This fungicidal activity is clinically important in severely immunocompromised patients who cannot rely on host immune defenses to clear the infection. In contrast, the azole antifungals are classified as fungistatic — they inhibit ergosterol synthesis and slow fungal growth but do not directly kill the organism. Amphotericin B's fungicidal classification applies broadly to its spectrum, not selectively to molds or yeasts.

Correct Answer

A — Echinocandin

Rationale

Caspofungin is classified as an echinocandin antifungal. Echinocandins inhibit beta-1,3-d-glucan synthase, the enzyme responsible for synthesizing beta-glucan in the fungal cell wall — a mechanism entirely distinct from the polyenes. Amphotericin B and nystatin are the polyene antifungals, which act at the fungal cell membrane by binding ergosterol. Fluconazole, itraconazole, and voriconazole are triazole antifungals, which inhibit fungal CYP51 (lanosterol 14-alpha-demethylase) to block ergosterol biosynthesis. Terbinafine is classified as an allylamine, which inhibits squalene epoxidase earlier in the ergosterol biosynthesis pathway.

Correct Answer

C — Formation of transmembrane pores that allow non-selective ion efflux

Rationale

When amphotericin B binds ergosterol in the fungal membrane, the drug molecules self-assemble around the sterol into barrel-shaped transmembrane pore structures. These pores allow non-selective ion flux, including outward potassium efflux down its concentration gradient. The resulting collapse of the membrane potential and failure of active transport systems leads to cell death. Inhibition of ergosterol biosynthesis at lanosterol demethylation is the mechanism of azole antifungals, not polyenes. Blockade of beta-1,3-d-glucan synthase is the mechanism of echinocandins. Tubulin cross-linking and mitotic spindle disruption describe the mechanism of griseofulvin.

Correct Answer

B — The drug is not absorbed from the gastrointestinal tract

Rationale

Amphotericin B deoxycholate is not absorbed from the gastrointestinal tract and must be administered intravenously for the treatment of systemic fungal infections. This pharmacokinetic property is the fundamental reason oral administration cannot achieve systemic therapeutic concentrations. The same property applies to nystatin, which shares this lack of gastrointestinal absorption and is therefore confined to topical and luminal applications. High plasma protein binding occurs after intravenous administration but is a distribution property, not an explanation for the failure of oral absorption. Amphotericin B is also not primarily eliminated by renal clearance and is not subject to significant first-pass hepatic metabolism — its intravenous requirement reflects absorption failure at the gut wall itself.

Correct Answer

C — High drug concentrations accumulate in the choroid plexus and meninges, allowing local antifungal activity despite low cerebrospinal fluid concentrations

Rationale

Although cerebrospinal fluid concentrations of amphotericin B are low after intravenous administration, the drug accumulates in high concentrations in the choroid plexus and in the meninges themselves. This tissue accumulation at the site of infection accounts for the drug's decades-long track record of efficacy in cryptococcal meningitis. The teaching point is that poor cerebrospinal fluid penetration does not prevent clinical efficacy when drug concentrations at the actual tissue site are adequate. Meningeal inflammation does increase blood-brain barrier permeability to some degree, but this alone does not account for the drug's efficacy — the primary explanation is choroid plexus and meningeal accumulation. Amphotericin B deoxycholate has no active transport mechanism across the blood-brain barrier, and liposomal carriers are a property of the liposomal amphotericin B formulation, not the deoxycholate preparation.

Correct Answer

D — Type 1 distal renal tubular acidosis with potassium and magnesium wasting

Rationale

Amphotericin B forms pores in the cholesterol-containing apical membranes of distal tubular epithelial cells. This direct tubular damage produces type 1 distal renal tubular acidosis, potassium wasting, and magnesium wasting. This component of nephrotoxicity is dose-dependent and cumulative. The other mechanism of amphotericin B nephrotoxicity — afferent arteriolar vasoconstriction — reduces glomerular filtration rate rather than increasing it, and is rapidly reversible. Proximal tubular dysfunction with glucosuria and aminoaciduria describes Fanconi syndrome, which is not the pattern produced by amphotericin B. Collecting duct obstruction is not a mechanism of amphotericin B nephrotoxicity.

Correct Answer

B — Renal vasoconstriction — saline loading attenuates the vasoconstrictive component of nephrotoxicity, preserving glomerular blood flow

Rationale

Saline loading primarily attenuates the vasoconstrictive component of amphotericin B nephrotoxicity. Amphotericin B causes afferent arteriolar vasoconstriction that reduces glomerular blood flow and decreases glomerular filtration rate — an effect that is rapidly reversible. Sodium loading blunts this vasoconstrictive response and helps maintain renal perfusion. Saline loading does not prevent the direct tubular damage produced by amphotericin B pore formation in apical tubular membranes — that component is dose-dependent, cumulative, and not meaningfully blocked by sodium delivery. Saline does not alkalinize the tubular lumen in a way that would block membrane insertion, nor does it act on both nephrotoxicity mechanisms simultaneously. Recognizing that the two mechanisms of amphotericin B nephrotoxicity differ in reversibility and susceptibility to prevention is the key teaching point of this question.

Correct Answer

A — Correct magnesium deficiency first — magnesium is required for normal distal tubular potassium reabsorption, so potassium repletion will remain ineffective until magnesium is restored

Rationale

In amphotericin B-associated electrolyte wasting, magnesium must be corrected before potassium repletion will be effective. Magnesium is required for normal function of the renal potassium-handling mechanism in the distal tubule. When magnesium is depleted, distal tubular potassium reabsorption is impaired, and administered potassium continues to be wasted in the urine regardless of how aggressively it is supplemented. Restoring magnesium levels first allows normal distal tubular potassium reabsorption to resume and makes potassium repletion effective. Correcting potassium first is the incorrect sequence — hypokalemia will remain refractory until the magnesium deficit is addressed. Simultaneous aggressive replacement without correcting magnesium first does not overcome the ongoing tubular potassium wasting. Discontinuing amphotericin B may ultimately be necessary for severe nephrotoxicity, but is not required as an immediate step before electrolyte correction, and for serious invasive fungal infections may not be a safe option.

Correct Answer

B — Mutations in ergosterol biosynthesis genes that deplete or structurally alter membrane ergosterol

Rationale

The primary mechanism of polyene resistance is depletion or structural alteration of ergosterol in the fungal cell membrane, resulting from mutations in ergosterol biosynthesis genes — particularly those encoding C-5 sterol desaturase and lanosterol 14-alpha-demethylase. By removing or altering the drug's binding target in the membrane, the organism survives despite the presence of amphotericin B. Efflux pump upregulation is the dominant resistance mechanism for azole antifungals, not polyenes, because polyenes act directly in the membrane rather than entering the cytoplasm. CYP51 mutations confer azole resistance, not polyene resistance. Cell wall thickening is not a recognized mechanism of polyene resistance.

Correct Answer

D — Drug-induced release of prostaglandins, interleukin-1, and tumor necrosis factor-alpha from monocytes and macrophages

Rationale

Amphotericin B infusion reactions are caused by drug-induced release of prostaglandins, interleukin-1, and tumor necrosis factor-alpha from monocytes and macrophages through toll-like receptor-dependent pathways, combined with complement activation. This is not an IgE-mediated allergic reaction, does not predict anaphylaxis, and does not contraindicate continued therapy. Because the mechanism involves prostaglandin release, premedication with acetaminophen attenuates the febrile component. Diphenhydramine addresses the histaminergic component. Meperidine breaks established rigors by a separate mechanism. The reaction typically diminishes in severity with subsequent infusions as tolerance develops.

Correct Answer

D — Continue amphotericin B with premedication using acetaminophen and diphenhydramine before each infusion, because the reaction does not indicate hypersensitivity and typically diminishes with subsequent doses

Rationale

Amphotericin B infusion reactions are caused by drug-induced release of inflammatory mediators from monocytes and macrophages — not by IgE-mediated hypersensitivity. Because this is not an allergic reaction, it does not predict anaphylaxis and does not contraindicate continued therapy. The correct management is to premedicate before each infusion with acetaminophen to attenuate the febrile response and diphenhydramine to address the histaminergic component. Meperidine can be used to break established rigors. Importantly, infusion reactions typically diminish in severity with subsequent infusions as tolerance develops, so the clinical course is the opposite of worsening sensitization. Switching to an azole would be appropriate for true drug hypersensitivity or for clinical failure, not for a predictable pharmacodynamic infusion reaction that can be managed with premedication.

Correct Answer

C — Switch to liposomal amphotericin B to reduce ongoing nephrotoxicity while maintaining antifungal coverage

Rationale

When a patient develops significant renal impairment during amphotericin B deoxycholate therapy, the appropriate response is to switch to a lipid formulation — most commonly liposomal amphotericin B — rather than discontinuing antifungal coverage in a patient with ongoing fungemia. Lipid encapsulation reduces the concentration of free amphotericin B available to interact with cholesterol in renal tubular membranes, substantially lowering the risk of further tubular damage while preserving the same broad-spectrum antifungal activity. Discontinuing all antifungal therapy in the setting of active fungemia would be clinically dangerous. Continuing amphotericin B deoxycholate at the same dose ignores the cumulative and partially irreversible nature of its direct tubular toxicity — waiting for creatinine to worsen further before acting is not appropriate management. Nystatin cannot be formulated for intravenous use at clinically safe concentrations and has no role in systemic invasive fungal infections.

Correct Answer

B — Nystatin cannot be administered intravenously at concentrations required for systemic antifungal activity without causing unacceptable toxicity

Rationale

The fundamental reason nystatin cannot be used for systemic fungal infections is toxicity, not spectrum. Nystatin has broad antifungal activity that includes Candida species, but at the blood concentrations that would be required for systemic efficacy it causes severe toxicity in humans. No safe intravenous formulation of nystatin is available. This is why nystatin is confined to topical and oral luminal use — it works at mucosal surfaces where it acts locally without requiring systemic absorption. Amphotericin B, the other polyene, was successfully formulated for systemic use because its delivery vehicle and dosing allowed a tolerable therapeutic window. Nystatin is fungicidal in vitro by the same membrane-pore mechanism as amphotericin B, so fungistatic vs. fungicidal classification does not explain its limitation. Nystatin does cover Candida species. Renal elimination and accumulation are not relevant because nystatin is not absorbed after oral administration and is not renally excreted at meaningful concentrations.

Correct Answer

D — Liposomal amphotericin B because lipid encapsulation limits free drug exposure to renal tubular cholesterol

Rationale

Liposomal amphotericin B is the preferred formulation when baseline renal impairment is present. Lipid encapsulation shields the drug from contact with mammalian cholesterol during circulation, reducing the concentration of free amphotericin B available to interact with cholesterol in renal tubular apical membranes — the site of the direct tubular toxicity. A baseline creatinine above 2.5 mg/dL is a standard indication to use a lipid formulation from the start rather than switching after nephrotoxicity develops, because tubular damage is cumulative and partially irreversible. Starting with amphotericin B deoxycholate at a reduced dose does not adequately protect against further tubular injury in a patient who already has significant renal impairment. Amphotericin B colloidal dispersion is associated with the highest rate of infusion reactions among lipid formulations and is rarely the first choice. Nystatin cannot be formulated for intravenous use at non-toxic concentrations and has no role in systemic invasive fungal infections.