Recent abdo/pelvic mri shows simple cyst left kidney. could it cause: ua over 18mth high pyuria, occasional haematuria, proteinuria & infection?

Kidney cyst: In a young person with your lab studies i would assume this is either a benign simple cyst or an abscess. See a urologist for a proper workup.

Simple cyst : Simple renal cysts are relatively common. If on MRI or contrast enhanced ct or even a good ultrasound, they meet the criteria for simple cyst, there's really no reason for concern. Solitary cysts of the kidney (of this nature) are not associated with the symptoms and findings you described.