Q. A 55-year-old white woman presents to your primary care clinic with complaints of persistent fatigue, weakness, and tingling in her fingers and toes. She has a medical history of Hashimoto's thyroiditis and vitiligo and has been on regular treatment for the past 10 years. Her temperature is 98.6°F (37°C), pulse is 86 beats per minute, blood pressure is 130/70 mmHg and respiratory rate is 14 per minute. Physical examination shows pallor and glossitis. Hypopigmented patches are noted on her lips and forearms. Neurological examination shows decreased deep tendon reflexes in both upper and lower limbs. Lab investigations are shown below. The peripheral blood smear shows macrocytic red blood cells and hypersegmented neutrophils. Given her clinical presentation, which of the following is the most probable cause for her condition?