If blue smoke is emitting from the exhaust, what is the most probable cause?

Blue smoke from the exhaust is most commonly associated with the burning of oil in the combustion chamber. This typically occurs when there is excessive oil entering the combustion process due to engine wear, particularly involving the piston rings. A worn piston ring can allow engine oil to bypass the rings and enter the combustion chamber, where it burns along with the fuel, resulting in the characteristic blue smoke.

In contrast, while a faulty head gasket might cause some symptoms related to engine performance, it is more commonly associated with white smoke due to coolant entering the combustion chamber rather than blue smoke. A broken valve spring, while it can lead to performance issues, does not typically cause blue smoke, as it primarily affects valve operation rather than oil consumption. A clogged catalytic converter would manifest primarily as a loss of power or efficiency, with exhaust appearing more dense rather than blue. Thus, the presence of blue smoke strongly indicates that the issue stems from worn piston rings, making this the most likely cause.