Sophia, a 10-year-old girl, is brought to the clinic by her parents due to poor growth, frequent falls, and tingling sensations in her hands and feet. Her parents report that she has difficulty seeing in low light and often bumps into objects. Her stools are pale, foul-smelling, and greasy. On examination, Sophia appears thin and has a clumsy, unsteady gait. Her vision is diminished, with evidence of retinitis pigmentosa, and she has reduced tendon reflexes and loss of vibratory sensation in her lower limbs. Laboratory findings reveal: Extremely low cholesterol and triglyceride levels. Acanthocytosis (thorny red blood cells) on peripheral smear. You suspect abetalipoproteinemia, a rare autosomal-recessive disorder caused by defective chylomicron and VLDL assembly due to MTTP gene mutation. Which of the following vitamins is characteristically deficient in patients with abetalipoproteinemia?
