Step 1: Sort the parasite into its group:
Strongyloides stercoralis is a nematode, which means a roundworm. The first job is to decide which drugs act on nematodes and which act on protozoa or bacteria.
The stool shows larvae, and the patient has been unwell for a long time. This fits strongyloidiasis, where the worm keeps re-infecting the same person through autoinfection.
Step 2: List what the four drugs are made for:
Nitazoxanide is an antiprotozoal drug that is also active against some worms. Its main use is in Giardia and Cryptosporidium infection.
Metronidazole is for anaerobic bacteria and for protozoa like amoeba, Giardia and Trichomonas.
Paromomycin is a poorly absorbed aminoglycoside that acts inside the gut lumen against amoeba and Cryptosporidium.
Ivermectin is a macrocyclic lactone (an anthelmintic) made for roundworm infections.
Step 3: Match the drug to the worm:
Only one of these four is a standard roundworm drug, and that is ivermectin.
Its mechanism is also a good fit. It opens glutamate-gated chloride channels in the invertebrate nervous system, so the worm is paralysed.
Human nerve cells do not have these channels in the same form, and ivermectin does not cross the blood-brain barrier easily. This is why the drug is safe for the patient.
Step 4: Confirm with the guideline:
Standard textbooks name ivermectin as the drug of choice for strongyloidiasis. A dose of 200 micrograms per kg is given once a day for 1 to 2 days.
In a person who is immunosuppressed, the course is repeated until the stool is clear. Albendazole is kept as a second line drug.
Step 5: Why the other three fail:
Nitazoxanide, metronidazole and paromomycin are chosen for protozoal and anaerobic infections.
None of them is trusted to clear the larvae or the adult worms of Strongyloides. So options (A), (B) and (C) are wrong.
Final Answer:
Ivermectin is the right drug for a Strongyloides stercoralis infection. This is option (D).