What is the primary cause of post-renal azotemia?

Post-renal azotemia occurs when there is an obstruction in the urinary outflow that leads to an accumulation of nitrogenous waste in the bloodstream. The primary cause of this condition is indeed urethral obstruction, which can prevent urine from being excreted from the bladder. This blockage can be caused by various factors such as urinary stones, tumors, or strictures.

When urine cannot be expelled, pressure builds up in the urinary system, affecting kidney function over time. The kidneys may still be producing urine, but it can't be eliminated effectively, leading to elevated levels of urea and creatinine in the blood, characteristic of azotemia.

In contrast, infection in the kidney, primary loss of nephrons, and severe dehydration are more closely associated with pre-renal and intrinsic renal causes of azotemia, rather than post-renal issues. Thus, understanding the mechanism of urinary obstruction is critical for identifying and managing post-renal azotemia effectively.