A 20-month-old boy is brought to the pediatric outpatient clinic by his mother, who noticed a white glow in his left eye on flash photography during a family gathering. On examination, the structure marked **A** (white pupillary reflex/leukocoria) is noted in the left eye, while the right eye shows a normal red reflex. The child has no history of prematurity, congenital cataract, or ocular trauma. Which of the following is the MOST CRITICAL NEXT STEP in management?