A microstrabismus is defined as a deviation less than ___ prism diopters that is not visible on the cover test.

• A. < 5 pd
• B. < 10 pd
• C. < 15 pd
• D. < 20 pd

Answer: (B)

Microstrabismus is a tiny, constant eye misalignment that is too subtle to be seen on a routine cover test. The standard clinical threshold used to define this level of misalignment is less than 10 prism diopters, so deviations under 10 pd are considered microstrabismic because they usually escape detection with the basic cover test and require more sensitive testing (like prism cover testing) to reveal them. Among the options, the one that states less than 10 prism diopters matches this definition, whereas smaller or larger thresholds would not align with how microstrabismus is defined in practice.