Hello Friend ..
Welcome to Health Care Magic..
I read your query and it seems that MCV is not been elevated because of Naproxen Pills . Naproxen Tablet is NSAIDS for your Knee pain
and it does not seems to affect your MCV unless you have decreased liver function .
Your doctor suggested you Ultrasound
to know the cause of elevated level of MCV in your blood .
Possibilities of elevated MCV are due to Vitamin B12
and Folic acid deficiency
which caused macrocytic anemia
Ultrasound will say the functioning capacity of LIVER and KIDNEY to know root cause of elevated MCV .
I Suggest you to have healthy diet along with vitamins and folic acid supplementations .
Wishing you a very healthy life.
Hope this information is helpful to you.
Thanks & Take Care.