You are answering a USMLE-style medical question. Question: A 44-year-old woman with hypothyroidism comes to the physician because of a 1-month history of tingling in her feet and poor balance. Her only medication is levothyroxine. Physical examination shows conjunctival pallor and an ataxic gait. Proprioception and sense of vibration are decreased in her toes bilaterally. Laboratory studies show macrocytic anemia and normal thyroid hormone levels. Histological evaluation of tissue samples obtained by esophagogastroduodenoscopy reveals atrophic changes of the gastric body and fundus with normal antral mucosa. Which of the following structures is most likely being targeted by antibodies in this patient? Options: A) Islet cell cytoplasm B) Deamidated gliadin peptide C) Parietal cells D) Smooth muscle Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.