Anthelmintic Agents

Correct Answer

B — Benzimidazole

Rationale

Mebendazole belongs to the benzimidazole class of anthelmintics, along with albendazole and thiabendazole. All benzimidazoles share a bicyclic benzene-imidazole scaffold and act by inhibiting tubulin polymerization in susceptible helminths. Macrocyclic lactones are represented by ivermectin. Nicotinic receptor agonists are represented by pyrantel pamoate, which causes depolarizing neuromuscular blockade. Prazinoisoquinolines are represented by praziquantel, which targets flukes and tapeworms.

Correct Answer

C — Macrocyclic lactone

Rationale

Ivermectin is a macrocyclic lactone — a class of large-ring lactone compounds derived from Streptomyces avermitilis fermentation. Macrocyclic lactones act by opening glutamate-gated chloride channels in invertebrate nerve and muscle tissue. Benzimidazoles (mebendazole, albendazole) act by inhibiting tubulin polymerization. Nicotinic receptor agonists (pyrantel pamoate) cause depolarizing neuromuscular blockade. Pentavalent antimonials target Leishmania and are not anthelmintic agents.

Correct Answer

A — Cestodes and trematodes

Rationale

Praziquantel is active against cestodes (tapeworms) and trematodes (flukes), including Schistosoma species, liver flukes, and most intestinal tapeworm infections. It is not effective against nematodes (roundworms), which lack the calcium channel targets through which praziquantel acts. Intestinal nematodes are treated with benzimidazoles or pyrantel pamoate. Filarial nematodes are treated with ivermectin or diethylcarbamazine. Praziquantel does not have meaningful activity across all helminth classes.

Correct Answer

D — Nicotinic acetylcholine receptor agonist

Rationale

Pyrantel pamoate is a nicotinic acetylcholine receptor agonist. It produces sustained depolarization at the helminth neuromuscular junction, causing spastic paralysis and expulsion of the worm from the gastrointestinal tract. Benzimidazole tubulin inhibition describes mebendazole and albendazole. Macrocyclic lactone chloride channel opening describes ivermectin. Prazinoisoquinoline calcium channel activation describes praziquantel. Each drug class acts through a distinct receptor or molecular target at the helminth neuromuscular junction or tegument.

Correct Answer

B — Albendazole, because it is substantially absorbed after oral administration and reaches therapeutic concentrations in systemic tissues

Rationale

The key distinction between the two benzimidazoles in clinical use is oral bioavailability. Albendazole is meaningfully absorbed from the gastrointestinal tract — absorption is further enhanced by a fatty meal — and its active sulfoxide metabolite circulates systemically, reaching larval cysts in the liver, lung, muscle, and central nervous system. Mebendazole is very poorly absorbed and acts almost exclusively within the intestinal lumen; it does not achieve systemic tissue concentrations sufficient to treat larval stages in tissues. This pharmacokinetic difference, not a difference in mechanism or potency, explains why albendazole is preferred for tissue-invasive helminthiasis.

Correct Answer

C — Filarial nematode infections such as lymphatic filariasis and tropical pulmonary eosinophilia

Rationale

Diethylcarbamazine is indicated for filarial infections, including lymphatic filariasis caused by Wuchereria bancrofti and Brugia species, and tropical pulmonary eosinophilia. It is also active against Loa loa but must be used cautiously in co-endemic areas due to encephalopathy risk. Intestinal nematodes (ascariasis, trichuriasis, hookworm) are treated with benzimidazoles or pyrantel pamoate. Cestode infections are treated with praziquantel or albendazole. Trematode infections are treated with praziquantel.

Correct Answer

A — They bind to parasite tubulin and inhibit its polymerization into microtubules, disrupting microtubule-dependent processes including glucose uptake, leading to parasite energy depletion and death

Rationale

Benzimidazoles bind selectively to parasite beta-tubulin and prevent its assembly into microtubules. Helminths depend on intact microtubules for glucose uptake across the intestinal epithelium; disruption of microtubule function deprives the parasite of its primary energy source, leading to immobilization and death over days. The selectivity for parasite tubulin over mammalian tubulin accounts for the favorable safety profile of these drugs at therapeutic doses. Glutamate-gated chloride channel opening describes ivermectin. Calcium permeability increase describes praziquantel. Nicotinic receptor agonism describes pyrantel pamoate.

Correct Answer

C — Albendazole is substantially absorbed from the gastrointestinal tract and its active metabolite circulates systemically, reaching larval cysts in tissues; mebendazole is poorly absorbed and acts only within the intestinal lumen

Rationale

The clinical difference between these two benzimidazoles is pharmacokinetic, not pharmacodynamic. Mebendazole is very poorly absorbed after oral administration — the vast majority of a dose remains in the intestinal lumen, where it acts against adult worms. Albendazole is meaningfully absorbed, and its active sulfoxide metabolite achieves systemic circulation, distributing into tissues including the liver, lung, muscle, and central nervous system where larval cysts reside. Taking albendazole with a fatty meal further improves absorption. Tissue-invasive helminthiasis — neurocysticercosis, echinococcosis, visceral larva migrans — requires a drug that reaches larval stages systemically, which only albendazole achieves. The difference is not due to differential tubulin affinity, gastric acid stability, or P-glycoprotein efflux.

Correct Answer

B — Ivermectin opens glutamate-gated chloride channels present in invertebrate nerve and muscle, causing hyperpolarization and paralysis; vertebrates lack these channels, making the drug selectively toxic to invertebrates

Rationale

Ivermectin binds to and opens glutamate-gated chloride channels — ion channels found in invertebrate nerve and muscle tissue that have no vertebrate equivalent. Channel opening increases chloride conductance, hyperpolarizing the cell membrane and producing flaccid paralysis of the parasite. Because vertebrates do not express glutamate-gated chloride channels, the drug has no equivalent pharmacological target in mammalian tissue, explaining its wide therapeutic window. At very high doses or in patients with compromised blood-brain barrier function, ivermectin can cause central nervous system toxicity in vertebrates through interaction with gamma-aminobutyric acid receptors — but this does not occur at therapeutic doses. Tubulin binding, cholinesterase inactivation, and calcium channel differences are not the basis for ivermectin's mechanism or selectivity.

Correct Answer

D — In areas co-endemic for Loa loa, patients with high Loa loa microfilaria burdens can develop encephalopathy when rapid killing of microfilariae in the central nervous system causes severe local inflammation

Rationale

The Loa loa safety constraint is one of the most important practical limitations of ivermectin mass drug administration. Loa loa is a filarial nematode co-endemic with onchocerciasis and lymphatic filariasis across central and West Africa. Patients with high Loa loa microfilaria burdens in the bloodstream are at risk for encephalopathy after ivermectin treatment because rapid killing of large numbers of microfilariae within central nervous system vessels triggers a severe local inflammatory response. This complication is rare but can be fatal. As a result, pre-treatment screening for Loa loa burden or use of alternative agents such as diethylcarbamazine is required before mass drug administration in co-endemic regions. The risk is not due to a neurotoxic metabolite, parasite disruption of the blood-brain barrier, or adult worm killing.

Correct Answer

A — Praziquantel increases calcium permeability in susceptible helminths, causing tetanic muscle contraction, tegument disruption, and exposure of parasite surface antigens to host immune attack

Rationale

Praziquantel acts by dramatically increasing calcium influx across the tegument of susceptible helminths. The resulting surge in intracellular calcium causes rapid tetanic muscle contraction and disrupts the structural integrity of the tegument — the parasite's outer surface layer. Tegument disruption exposes surface antigens previously hidden from the host immune system, allowing antibody-mediated and cellular immune mechanisms to participate in parasite killing. The combination of physical tegument damage and immune-mediated attack produces rapid worm death. Praziquantel does not inhibit tubulin, open glutamate-gated chloride channels, or act at nicotinic receptors — those are the mechanisms of benzimidazoles, ivermectin, and pyrantel pamoate respectively.

Correct Answer

C — Rifampin induces hepatic drug-metabolizing enzymes, accelerating praziquantel metabolism and reducing praziquantel plasma levels below therapeutic concentrations

Rationale

Rifampin is one of the most potent inducers of hepatic drug-metabolizing enzymes in clinical use. When praziquantel is co-administered with rifampin, hepatic metabolism of praziquantel is markedly accelerated, reducing praziquantel plasma concentrations — in some cases to subtherapeutic levels — and risking treatment failure for the helminth infection. This interaction is clinically relevant because schistosomiasis and tuberculosis co-occur in overlapping geographic regions. When rifampin-based tuberculosis treatment cannot be deferred, praziquantel dose adjustment or an alternative anthelmintic strategy may be required. The interaction is entirely pharmacokinetic — rifampin does not block praziquantel at its target, praziquantel does not affect rifampin absorption, and neither drug causes clinically significant nephrotoxicity at standard doses.

Correct Answer

B — Pyrantel pamoate acts as a nicotinic acetylcholine receptor agonist at the helminth neuromuscular junction, producing sustained depolarization and spastic paralysis followed by expulsion

Rationale

Pyrantel pamoate mimics acetylcholine at nicotinic receptors on the helminth neuromuscular junction. Unlike acetylcholine, it is not broken down by cholinesterase, so its depolarizing action is sustained. Persistent depolarization produces spastic paralysis of the worm, which is then expelled by normal intestinal peristalsis. Because pyrantel pamoate is poorly absorbed from the gastrointestinal tract, it acts locally within the intestinal lumen with minimal systemic exposure and a favorable safety profile. Glutamate-gated chloride channel opening describes ivermectin. Tubulin inhibition describes benzimidazoles. Calcium permeability increase describes praziquantel.

Correct Answer

D — Diethylcarbamazine immobilizes microfilariae and alters their surface properties, enhancing recognition and destruction by the host immune system rather than acting by direct toxicity alone

Rationale

Diethylcarbamazine's mechanism is distinct from the other anthelmintics in this chapter in that it does not act primarily through direct parasite toxicity. Instead, it immobilizes microfilariae and alters properties of their surface, making them more susceptible to recognition and destruction by host immune effector cells. This immune-dependent mechanism explains why diethylcarbamazine is more effective against microfilariae than against adult worms, and why it is less effective in immunocompromised hosts. It does not open glutamate-gated chloride channels, inhibit tubulin, or generate reactive oxygen species as its primary mechanism.

Correct Answer

B — Immunosuppression permits unchecked larval amplification; filariform larvae penetrate the bowel wall and carry intestinal bacteria into the bloodstream, causing gram-negative septicemia — the hyperinfection syndrome that ivermectin treatment before immunosuppression is designed to prevent

Rationale

Strongyloides stercoralis is unique among intestinal helminths in its ability to complete its entire life cycle within a single host through autoinfection. In immunocompetent individuals, this autoinfection cycle is kept in check by immune surveillance. When T-cell immunity is suppressed — as in this patient receiving tacrolimus and high-dose corticosteroids after transplantation — larval amplification proceeds unchecked, producing massive numbers of filariform larvae. These invasive larvae penetrate the intestinal wall and carry intestinal bacteria with them into the mesenteric lymphatics and systemic circulation, causing gram-negative septicemia and meningitis. This is the hyperinfection syndrome, and it carries high mortality. The bacteria are physically transported by the larvae through the bowel wall — not released as endotoxin, not activated by tacrolimus, and not entering through tight junctions.

Correct Answer

A — Loss of T-cell immune control would allow autoinfection to amplify unchecked, potentially producing hyperinfection syndrome with larval dissemination, bowel wall penetration, and life-threatening gram-negative septicemia; pre-treatment with ivermectin eliminates this risk

Rationale

Strongyloides stercoralis can persist for decades in a human host as a low-level, asymptomatic infection held in check by intact T-cell immunity. When that immunity is removed by transplant immunosuppression, the autoinfection cycle accelerates without limit. The resulting hyperinfection syndrome causes larval invasion of the bowel wall and dissemination to lungs, liver, central nervous system, and other organs, with concurrent transport of intestinal bacteria into the bloodstream. The outcome is gram-negative septicemia and meningitis with high mortality. This is why all candidates for organ transplantation who have lived in or traveled to Strongyloides-endemic regions must be screened and treated before immunosuppression begins — a course of ivermectin clears the infection and prevents this complication. Immunosuppressive drugs do not stimulate egg production, the transplanted organ is not a site of larval deposition, and Strongyloides does not remain confined to the intestinal lumen under immunosuppression.

Correct Answer

D — Fasciola hepatica is intrinsically resistant to praziquantel; triclabendazole is the drug of choice for this infection

Rationale

Praziquantel is highly effective against most trematodes and cestodes, but Fasciola hepatica is a well-established exception — it is intrinsically resistant to praziquantel at clinically achievable concentrations. The mechanism of this resistance is not fully characterized, but it is a consistent finding that distinguishes Fasciola from other liver flukes such as Clonorchis sinensis and Opisthorchis species, which do respond to praziquantel. The correct treatment for Fasciola hepatica infection is triclabendazole, a benzimidazole derivative with a distinct mechanism that is active against Fasciola at both larval and adult stages. Recognizing this exception is clinically important because empirical praziquantel therapy for presumed liver fluke disease will fail when Fasciola is the causative organism. Biliary inactivation, efflux transporters, and a requirement for hepatic metabolic activation are not the explanation for praziquantel's failure against Fasciola.

Correct Answer

C — Rapid killing of large numbers of microfilariae releases parasite antigens that trigger a host inflammatory response; the reaction reflects the parasite burden, not drug toxicity

Rationale

The Mazzotti reaction is a systemic inflammatory response that occurs when diethylcarbamazine rapidly kills large numbers of Onchocerca volvulus microfilariae. The sudden release of parasite antigens from dying microfilariae triggers host immune and inflammatory mediator release, producing fever, pruritus, rash, and hypotension — a reaction whose severity is proportional to the microfilarial burden rather than to the drug dose. It is not a drug allergy, not direct mast cell toxicity, and not a toxic metabolite effect. Management is with antihistamines and corticosteroids to blunt the inflammatory response; the reaction is not a reason to discontinue diethylcarbamazine, and it is expected to be less severe with subsequent doses as microfilarial burden decreases.