Focal renal necrosis is caused by acute renal failure. The etiology of acute renal failure is diverse and can be divided into three categories: pre-renal, renal and post-renal. Common causes of prerenal ARF include reduced blood volume (e.g., fluid loss and bleeding from various causes), reduced effective arterial blood volume, low cardiac output, intrarenal hemodynamic changes (including renal vasoconstriction and dilation imbalance), and mechanical obstruction of the renal arteries. The etiology of post-renal acute renal failure is primarily acute urinary tract obstruction. Renal acute renal failure is defined as renal parenchymal injury, commonly due to renal ischemia or nephrotoxic substances that damage tubular epithelial cells (e.g., acute tubular necrosis, ATN), but also includes the dramatic decline in renal function associated with glomerular disease, renal vascular disease, and interstitial lesions. The etiology of ATN varies with the patient population and the environment in which it occurs. ATN is often triggered by infections, various factors that lead to a decrease in effective circulating volume or blood pressure, and various nephrotoxic drugs. The susceptible population for the development of ATN includes patients with underlying renal disease, hypertension, diabetes, cardiovascular disease, and advanced age.