infant aged now 4 m, he had circular lesion in back of his head 2 cm in diameter, red, with scales on it , hair is preserved

is it tinea ???

If the baby is 4 months old - the hairloss may well be tinea capitis, caused by a fungal infections.

http://aapgrandrounds.aappublications.org/content/7/5/57.extract

little information for skin condition can u place apicture may not be cutis aplasia a fungal infection will be easily diag by dermatolgical oppinnion

Depending on how long the lesion has been present, more often than not, you will have hair loss. The hairs in the infected area are dull and brittle and fall out easily. Scrape some of the scale onto a slide and perform a KOH prep and look for hyphae to help with diagnosis of tinea capitus.