**Core Concept**
Gastrectomy, the surgical removal of part or all of the stomach, can lead to significant metabolic complications due to altered digestive and absorptive functions. The most common complications are related to vitamin and mineral deficiencies, particularly those affecting the hematopoietic, nervous, and musculoskeletal systems.

**Why the Correct Answer is Right**
The most common metabolic complication of gastrectomy is vitamin B12 deficiency. This is because the stomach plays a crucial role in the production of intrinsic factor, a protein necessary for the absorption of vitamin B12. After gastrectomy, patients are at risk of developing pernicious anemia, a condition characterized by a deficiency in vitamin B12. The lack of intrinsic factor impairs the absorption of vitamin B12 from food, leading to a decrease in the levels of this essential vitamin.

**Why Each Wrong Option is Incorrect**
**Option A:** While iron deficiency is a common complication of gastrectomy, particularly in patients who have undergone partial gastrectomy, it is not the most common metabolic complication. This is because the stomach's role in iron absorption is less critical than its role in vitamin B12 absorption.
**Option B:** Electrolyte imbalances can occur after gastrectomy, but they are not the most common metabolic complication. The stomach plays a role in electrolyte regulation, but this is not the primary concern after gastrectomy.
**Option C:** Malnutrition is a potential complication of gastrectomy, but it is a broader term that encompasses multiple metabolic complications. Vitamin B12 deficiency is a more specific and common complication.

**Clinical Pearl / High-Yield Fact**
To prevent vitamin B12 deficiency in patients who have undergone gastrectomy, it is essential to administer vitamin B12 supplements regularly. This is particularly important in patients who have undergone total gastrectomy, as they are at highest risk of developing pernicious anemia.

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