Q. A 5-year-old girl is brought to the pediatric clinic by her parents due to poor coordination and balance. They report that she often stumbles and falls while playing, and her motor skills are significantly behind those of her peers. Her history reveals that she was diagnosed with a severe malabsorption disorder in infancy, but she has not taken any specific vitamin supplements. On physical examination, she shows signs of muscle weakness and an unsteady gait, but no sensory changes. Laboratory studies reveal anemia, and a peripheral blood smear demonstrates acanthocytes. No megaloblastic changes are seen on the blood smear, and the serum methylmalonic acid level is normal. What is the deficient vitamin in this child's case?