The ARK (also spelled Ark) is a barrel attachment found in the Extinction maps Awakening and Exodus. It allows weapons to turn into a laser weapon, though it still uses conventional ammunition and also has the additional effect of causing extra damage to cryptids, which further makes any that are killed with the weapon it's attached to explode in a similar fashion as to the Pipe Bombs.
A kidney, ureter, and bladder (KUB) study is an X-ray study that allows your doctor to assess the organs of your urinary and gastrointestinal systems. Doctors can use it to help them diagnose urinary disorders and causes of abdominal pain.
A kidney stone is a hard, crystalline mineral material formed within the kidney or urinary tract. Kidney stones are a common cause of blood in the urine (hematuria) and often severe pain in the abdomen, flank, or groin. Kidney stones are sometimes called renal calculi. The condition of having kidney stones is termed nephrolithiasis.