What is a likely cause of two cylinders reading 50 PSI in a V-8 engine while others read between 135-140 PSI?

Two cylinders reading 50 PSI while the others show readings between 135-140 PSI suggests a significant issue affecting those specific cylinders' ability to maintain compression. A blown head gasket is a plausible reason for this scenario. When a head gasket fails, it can create a breach between the cylinders or between a cylinder and the coolant passages. This results in a loss of compression in the affected cylinders, which would explain why they are not holding the expected PSI levels.

Additionally, because the other cylinders maintain their compression, it indicates that the problem is localized rather than a systemic issue across the entire engine. A blown head gasket can also allow coolant to enter the combustion chamber, leading to other issues such as overheating or steam in the exhaust, which can further verify the diagnosis if those symptoms are present.

The other options relate to valve issues or gasket failures, but they wouldn't typically cause such a significant drop in compression across two specific cylinders without impacting adjacent cylinders or the whole engine. Hence, a blown head gasket is the most likely explanation for the observed compression disparity.