A 3-year-old boy is brought to the pediatric ophthalmology clinic by his mother with a complaint of inward turning of the left eye. The child was born full-term without any perinatal complications. On examination, the left eye is turned inward (esotropia). The cover test reveals that when the right eye is covered, the left eye moves outward to fixate, and when the left eye is covered, the right eye moves inward to take up fixation. The child has full extraocular movements bilaterally. Cycloplegic refraction shows +2.50 D sphere in both eyes. What is the most likely diagnosis?