Question 256

Question:

A 12-year-old boy has a pruritic, coin-shaped, scaly erythematous lesion with a raised border and central clearing on his right arm. Potassium hydroxide preparation results are consistent with tinea corporis. A drug with which of the following mechanisms of action is most likely to be effective?

Correct Answer: A.  Blockade of ergosterol synthesis

Tinea corporis or ringworm is a fungal infection. An effective treatment for this infection is a topical azole that acts by blocking ergosterol synthesis and thereby increasing fungal membrane permeability. Miconazole and clotimazole are two azoles available in topical solutions that can be used to treat this infection.

Inhibition of DNA synthesis is the mechanism of action of flucytosine, which converts to fluorouracil and competes with uracil. This drug can be used to treat systemic fungal infections, but the side effects of nausea, vomiting, diarrhea, and bone marrow suppression limit its use.

Inhibition of ribosomal protein synthesis is the mechanism of action of various antibacterial agents including aminoglycosides, tetracyclines, chloramphenicol, erythromycin, Lincomycin, and Clindamycin. These drugs are used to treat various bacterial infections, but they are not used to treat fungal infections.

Interference with mycolic acid synthesis is how Isoniazid works. Isoniazid is a drug used for tuberculosis prophylaxis and in combination with other drugs for therapy for active tuberculosis. Isoniazid is not used to treat fungal infections.

Rifampin is another drug used in combination with Isoniazid for active tuberculosis therapy. It works by irreversibly binding to and inhibition of DNA-dependent RNA polymerase. Rifampin also is used for meningococcal prophylaxis, but it is not used to treat fungal infections.