I agree it does resemble the Mahonia aquifolium, or Oregon Grape.. but the leaves are not like holly leaves as you can see from the photo, they are smooth edged. Maybe this is a hybrid? I posted a larger picture, I hope it comes out bigger now. Here is a link that might be useful:

I have been growing meiwa kumquats from seed. They are consistently attacked by spider mites. I believe that the roots do not change their nutrient formulation to defend itself against pest and this is why they are near impossible to grow from seed. I am curious if you have picture of your tree when it was doing very well. Has it produce fruit yet. Al my kumquat tree have died. They put on a growth spurts of significance harden off loose all the leave and die back to 6 inches and then put on beautiful growth. Harden off then infestation takes of every leaf. This stripping growing process happens 2 time a year and my tree went from 8 inches to 30 inches back to 8 they grow to 30 and die back to 8. Steve

no photo.

if i had a window like that .. i would wonder how cold it might be in my MI ... in winter ... and i would consider that variable in plant choice .. for you ... where are you ... big city name ... and how about a pic of your room.. it would be best to make suggestions on your site.. rather than this random www pic ... ken ps: the thing that jumped out for me in the pic.. is all that glorious light ... and the darn thing is leaning ... one would think it would be bolt upright.. simply by turning it once every 3 months... which then made me think.. staged or photoshopped ... and then i lost interest ... because its fake ... and then i wondered what your situation was .. and then i decided i needed to stop drinking coffee because apparently i cant stop typing.. lol ... which is usually the case .. when the ps is longer than the original reply ... lol.. enough already ....