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I don't think its a K deficiency. Plants need lots of K in later flowering stages. Plants like to absorb nutrients from leaves beneath the plant when they are having all kinds of deficiencies. Since other leaves are still looking green, i don't think its a CAL-MAG problem, most of the growers get those a few weeks later when not given any. What you could do is flush your plant a little bit and check your run-off. Check the PH and the EC. If you have the wrong PH, your plant doesn't use the giving nutrients well, if you have a low ec like 0.7-1, you are not giving enough nutrients. I don't know if you have something like enzymen? It is very usefull and it will make sure old nutrients inside your medium will get into use again. Some people still like to give their plants vegative nutrients for some weeks in flowering state and since you are still in your pre-flower, you should. I also would recommend to get rid of that leave, nothing good will come of leaving it on. Goodluck growing :)