A 52-year-old woman with Hashimoto thyroiditis comes to her physician because of six months of progressive fatigue and unsteadiness when walking. She has no dietary restrictions and eats meat regularly. On examination, vibration sense is decreased at the ankles bilaterally and she sways significantly with eyes closed. Laboratory studies show a hemoglobin of 8.4 g/dL, MCV of 112 fL, hypersegmented neutrophils on peripheral smear, and serum B12 below the detectable limit. Serum gastrin is markedly elevated. Biopsy of the gastric body shows atrophy of the fundic glands with lymphocytic infiltration and replacement of the glandular epithelium by mucous-type cells.
Autoimmune destruction of which gastric cell type most directly accounts for this patient's clinical presentation?