Hydronephrosis is best described as what mechanism?

• A. Obstruction increases pressure in the renal pelvis and calyces causing dilation
• B. Decreased renal blood flow
• C. Inflammation of glomeruli
• D. Amyloidosis

Answer: (B)

Hydronephrosis results when urine outflow is blocked, causing urine to back up and raise pressure inside the kidney proximal to the blockage. That increased pressure distends the renal pelvis and calyces, producing dilation. If the obstruction is chronic, this pressure can eventually thin the renal parenchyma and impair function. This mechanism is different from decreased renal blood flow (ischemia), inflammation of the glomeruli (glomerulonephritis), or amyloid deposition (amyloidosis), which cause kidney problems through distinct processes rather than obstruction-induced dilation.