A 6-year-old girl is referred to the strabismus clinic with a 2-month history of sudden-onset left eye inward deviation. The mother reports that the child was playing with her siblings when she suddenly complained of double vision and the left eye turned inward. On examination, the left eye is deviated inward (esotropia) with restricted abduction. The right eye abducts normally. Pupillary reactions are normal and symmetric. Cycloplegic refraction shows no significant refractive error (−0.25 D OU). The child has no history of head trauma or recent illness. What is the most likely diagnosis?