Thanks for writing in.
The brown spot you are talking about can be onychomycosis
or fungal infection
Oral and topical antifungals are treatment of choice.You can apply NAILON lotion over the nail.
Please visit a good dermatologist
to confirm the diagnosis and starting on oral antifungal
medications.Treatment of nail fungal infection should be taken for 6 months.It has slow effect but it is curable.
Hope this helps.