A combination of crystal precipitates (most typically struvite) and protein matrix (mucus and/or blood cells) usually produced in excess after an acute episode of feline cystitis (bladder inflammation) can coalesce to form a urethral plug and cause a complete blockage of the urethra, preventing urination. Inflammation of the bladder wall can cause the protein matrix to "leak" from the wall. Even without crystal formation, a thick protein matrix may cause urethral obstruction by itself though this is seen more rarely. Urethral plugs are described as “mechanical” blocks of the urethra.