**Core Concept**
Gastrectomy, the surgical removal of all or part of the stomach, can lead to various metabolic complications due to changes in stomach anatomy and function. The most common metabolic complication of gastrectomy is related to vitamin B12 deficiency, which can occur due to impaired intrinsic factor production in the stomach.

**Why the Correct Answer is Right**
Intrinsic factor, a glycoprotein produced by the parietal cells of the stomach, is essential for vitamin B12 absorption. Gastrectomy can lead to a decrease or complete loss of intrinsic factor production, resulting in malabsorption of vitamin B12. This deficiency can cause megaloblastic anemia, neurological symptoms, and other systemic complications. The impaired production of intrinsic factor is a direct consequence of the loss of parietal cells during gastrectomy.

**Why Each Wrong Option is Incorrect**
**Option A:** While weight loss and malnutrition are potential complications of gastrectomy, they are not the most common metabolic complications. Weight loss is often a result of reduced food intake and altered digestive processes.
**Option B:** Hypocalcemia is not a direct metabolic complication of gastrectomy. Although patients with gastrectomy may experience changes in calcium absorption, hypocalcemia is not the most common metabolic issue.

**Clinical Pearl / High-Yield Fact**
Gastrectomy patients require lifelong vitamin B12 supplementation to prevent deficiency and its associated complications. It is essential to monitor their vitamin B12 levels regularly and adjust the dosage as needed.

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