A 5-year-old boy from rural Bihar presents with his father complaining of inability to see in the evening and difficulty recognizing faces in dim light for the past 4 weeks. On examination, the anterior segment appears normal under bright illumination, but the father reports the child bumps into objects at dusk. Dark adaptation testing shows prolonged recovery time. Fundoscopy is unremarkable. The child's diet consists mainly of rice and lentils with rare access to fruits or vegetables. What is the earliest clinical sign of vitamin A deficiency evident in this case?