Royal Canin Feline Urinary SO Moderate Calorie Dry (3.3 lbs) Effectivley Dissolves pure struvite urolith crystals Moderate calorie content, compared to feline Urinary SO to help maintain ideal weight. Increasing the urine volume simultaneously reduces the saturation of urine with calcium oxalate and struvite crystals Relative Super saturation or RSS methodology predicts the crystallization potential of urine. RSS is used to develop diets that control both struvite and calcium oxalate urolithiasis