**Core Concept**
The question is testing the knowledge of a rare but serious complication of partial gastrectomy, a surgical procedure that involves the removal of a part of the stomach. This complication is related to the malabsorption of a specific vitamin due to the altered anatomy of the gastrointestinal tract.

**Why the Correct Answer is Right**
The most probable diagnosis in this case is Vitamin B12 deficiency. This is because the stomach lining, where intrinsic factor is produced, is partially removed during the surgery. Intrinsic factor is essential for the absorption of Vitamin B12 in the terminal ileum. Without it, patients can develop megaloblastic anemia, neurological symptoms, and even dementia. The lack of intrinsic factor leads to impaired absorption of Vitamin B12, which is crucial for the synthesis of myelin, the fatty substance surrounding nerve fibers.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while malabsorption of other nutrients can occur after partial gastrectomy, Vitamin B12 deficiency is the most common and clinically significant complication.

**Option B:** This option is incorrect because pancreatic insufficiency is more commonly associated with pancreatic disease or resection, rather than partial gastrectomy.

**Option C:** This option is incorrect because iron deficiency anemia is a common cause of anemia, but it is not directly related to the neurological symptoms in this context.

**Clinical Pearl / High-Yield Fact**
To remember the association between partial gastrectomy and Vitamin B12 deficiency, recall that intrinsic factor is produced in the stomach lining, which is removed during the surgery. This leads to impaired absorption of Vitamin B12, resulting in neurological symptoms.

**Correct Answer:** D. Vitamin B12 deficiency.