Antibiotics and Chemotherapy Questions and Answers

Answer: B

Atovaquone and proguanil hydrochloride are antimalarial drugs.

The fixed-dose combination is used both to treat uncomplicated falciparum malaria and to prevent malaria in travellers. Atovaquone blocks electron transport in the parasite's mitochondria, and proguanil, through its active metabolite cycloguanil, inhibits the parasite's dihydrofolate reductase. The two act together on different targets.

They are not antivirals or antibiotics. Atovaquone alone is also used against Pneumocystis pneumonia, but the atovaquone-proguanil combination is classed as an antimalarial, not an antifungal.

Answer: D

Methotrexate is a folic acid antagonist.

It inhibits dihydrofolate reductase, the enzyme that regenerates tetrahydrofolate. Without tetrahydrofolate, cells cannot make thymidine and purines for DNA synthesis, so rapidly dividing cells are hit hardest. It is used in cancers, rheumatoid arthritis and psoriasis, and folinic acid (leucovorin) rescue counters its toxicity.

Purine antagonists are drugs such as 6-mercaptopurine. Alkylating agents such as cyclophosphamide and busulfan cross-link DNA. Anticancer antibiotics include doxorubicin and bleomycin.

Answer: B

Busulfan is an alkylating agent.

Busulfan is an alkyl sulphonate. It attaches alkyl groups to DNA and cross-links its strands, which stops cell division. It acts strongly on the bone marrow, so it is used in chronic myeloid leukaemia and to condition patients before bone marrow transplantation.

It is not an antibiotic or an antiviral. Antimetabolites such as methotrexate and 5-fluorouracil work differently, by mimicking the building blocks of DNA.

Answer: B

Streptomycin is a bactericidal drug.

Streptomycin, an aminoglycoside antibiotic and the first drug used against tuberculosis, binds to the bacterial ribosome and causes faulty protein synthesis. This kills the bacteria outright rather than merely stopping their growth.

A bacteriostatic drug, such as tetracycline, only halts multiplication. 'Germicidal' is a general term for disinfectants that kill germs on surfaces. Fungicidal drugs act on fungi, against which streptomycin is not used.

Answer: C

The keyed answer is rifampicin.

Rifampicin is a first-line anti-tubercular drug that kills Mycobacterium tuberculosis by blocking its RNA synthesis. It is strongly bactericidal and also sterilises slowly multiplying bacilli, which makes it the backbone of short-course treatment.

Ethambutol and thioacetazone are bacteriostatic: they stop the bacilli multiplying without killing them. Ofloxacin is also bactericidal, but it is a second-line drug kept for drug-resistant tuberculosis, so the first-line bactericidal drug among the options is rifampicin.

Answer: C

Ketoconazole is used as an antifungal.

It is an imidazole antifungal. It inhibits the fungal cytochrome P450 enzyme lanosterol 14α-demethylase, blocking the synthesis of ergosterol, the main sterol of fungal cell membranes. It is used, mostly topically today, for skin and scalp infections such as tinea and seborrhoeic dermatitis.

It has no useful activity against bacteria, Mycobacterium tuberculosis or the malaria parasite, so the other three uses are wrong.