When i am standing and try to fully extend my left leg, i get a sharp pain in my knee. what is wrong and is it serious?

Tendonitis/Meniscus: It depends on the location of the pain and whether or not there was any injury or overuse. The most common cause of anterior knee pain with extension is tendonitis, an inflammation of the tissue attachments above and below the kneecap. In the absence of other signs like swelling or warmth , treatment with rest and ice is often useful.

Need more info: Like where in the knee? If it is in the front could be the knee cap and arthritis or tendonits or impingement of the fat pad. If it is on the inside could be a meniscus tear or mcl strain, outside could be it band tendonitis, lcl strain or lateral meniscus tear. Posterior could be meniscus tear, hamstring or gastroc tendonitis or even sciatica.