What can cause red thick line beneath the thumbnail? – Dr. Aruna Prasad

Red thick line under the thumb or any nail, have to see to understand what you are asking in the query. Probably it is splinter hemorrhages. Splinter hemorrhages are reddish lines which occur under the nails. This is because of the capillary vasculature, which is the thin blood vessels which supply the skin and the nails. So maybe there is a disruption of the capillaries that gives rise to seepage of blood under the nails or under the skin. There are reasons for this. It could be trauma to the nail or the nail bed can cause splinter hemorrhages. Systemic conditions like chronic renal failure, skin conditions like psoriasis, any long eczema, or dermatitis, langhan’s cell disease. All these can cause splinter hemorrhages. Meet a dermatologist, they will be able to diagnose and treat you accordingly.