Assertion: Griseofulvin extracted from P. griseofulvum is used for ringworm treatment.
Reason: Trichophyton, Epidermophyton, etc. cannot grow well in presence of Penicillium griseofulvum.

Both Assertion (A) and Reasoni (R) are correct and R is the correct explanation of A.

Antibiotics are chemical substances produced by some microbes which in small concentration can kill or retard the growth of harmful microbes without adversely affecting the host. Griseofulvin is an antibiotic produced by Penicillum griseofulvum and is antifungal in nature, especially effective against ringworm. Ringworm is caused by dermatophytic fungi like Epidermophyton, Trichophyton, Microsporium, etc. which cannot grow in presence of P. griseofulvum because of the antagonistic effect of antibiotic griseofulvin secreted by it.