Which enzyme is least useful for differentiating necrotic from obstructive jaundice?

The enzyme that is least useful for differentiating necrotic from obstructive jaundice is LD, or lactate dehydrogenase. While LD is a valuable enzyme in assessing tissue damage and can be elevated in various types of liver disease, it is not specific enough to differentiate between the causes of jaundice.

In the context of obstructive jaundice, which usually results from bile duct obstruction leading to increased levels of bilirubin and specific liver enzymes, LD levels may also rise due to liver cell damage. However, LD is also elevated in other conditions unrelated to liver obstruction or damage, making it a less reliable marker for distinguishing between necrotic and obstructive jaundice.

In contrast, other enzymes such as GGT, ALT, and 5' nucleotidase have greater specificity in this scenario. For instance, GGT is particularly elevated in cholestatic diseases, while ALT is more indicative of hepatocellular injury. 5' nucleotidase is also associated with obstructive jaundice and helps reinforce the diagnosis.

Thus, LD is valuable in providing information about tissue and cell damage, but its general elevation across multiple conditions limits its utility for making specific differentiations between necrotic and obstructive jaundice.