Hi doctor i want ask you something before how many...

Dear Customer
Welcome, and thank you for trusting us
Regarding what you mentioned, the  most common cause of yellow discoloration of the nail is the fungal infection of the nail.

What are the other possible causes ?

This may be due to other causes such as thyroid condition , diabetes mellitus or psoriasis.

What should you do in this case ?

WE advise you to visit your GP  doctor so that the doctor could do a full clinical evaluation of the case then he will treat it accordingly.
  Stay well.   
Feel free to message me anytime here if you have further questions.
It is our pleasure to help. Please feel free to reach out to me anytime.
You can also ask our other certified experts in various fields such as Nutritionists, Lawyers, IT Experts, Therapists, Mechanics, Doctors, Handymen, and more.
Kind Regards