Why does urine becomes dark in post hepatic jaundice?

In post-hepatic jaundice, the obstruction occurs beyond the liver, usually in the common bile duct or the ampulla of Vater. This blockage prevents bile from flowing into the duodenum, leading to several consequences.

One of the primary effects of bile duct obstruction is the accumulation of bilirubin in the bloodstream. Bilirubin is a yellow pigment that is formed as a byproduct of the breakdown of red blood cells. Normally, bilirubin is transported to the liver, where it is conjugated with glucuronic acid to form water-soluble bilirubin glucuronide. This conjugated bilirubin is then excreted in the bile and eventually into the feces.

In post-hepatic jaundice, the obstruction in the bile duct prevents the conjugated bilirubin from being excreted into the feces. As a result, bilirubin accumulates in the blood, leading to a condition called hyperbilirubinemia.

The presence of excess bilirubin in the bloodstream causes the yellowing of the skin, eyes, and mucous membranes, which is the characteristic sign of jaundice. Additionally, the high levels of bilirubin can also affect the color of urine.

Normally, urine is light yellow in color due to the presence of a pigment called urobilin. Urobilin is a breakdown product of bilirubin that is excreted in the urine. However, in post-hepatic jaundice, the obstruction in the bile duct prevents urobilin from reaching the intestines. As a result, the urine becomes dark or amber-colored due to the absence of urobilin.

Therefore, the dark urine in post-hepatic jaundice is a consequence of the obstruction in the bile duct, leading to the accumulation of bilirubin and the absence of urobilin in the urine.