The drug of choice (D.O.C) for treating isospora infections, a type of parasitic infection caused by Isospora belli, is Co-trimoxazole. This condition, known for causing chronic diarrhea and affecting individuals with impaired immune systems, particularly AIDS patients, requires an effective antimicrobial agent. Co-trimoxazole, a combination of sulfamethoxazole and trimethoprim, is the recommended treatment due to its specific effectiveness against the protozoan parasites causing isospora infections. Therefore, among the options provided:
- Penicillin G
- Benzathine penicillin
- Co-trimoxazole
- Albendazole
Co-trimoxazole is the correct choice, making it the best-suited medication for this condition among the given alternatives.