15-20% of FLUTD cases are caused by uroliths ("stones") which occur when crystals which may be present in urine coalesce to form small stones. The most common types observed are struvite and calcium oxalate. The majority of uroliths are located in the urinary bladder, but can also form in the kidneys, ureters and urethra. The underlying microscopic crystals which go on to form bladder stones are not on their own an abnormal finding. They are often found in well-hydrated healthy cats who simply flush them out with their urine. However, if they accumulate in the bladder and are allowed to clump together to form larger stones (ie uroliths) which then become lodged in the urethra or alternatively, coalesce with inflammatory bladder debris or “matrix” to form § urethral plugs following an attack of feline cystitis (see below), then they could pose a risk of possible obstruction. If the obstruction (whatever the cause) prevents urination altogether then it is a medical emergency which requires immediate intervention.