Topics:
Animal Physiology, Animal Refresher
Extrinsic Castle (Gastric) Factor
Extrinsic Castle (gastric) factor is:
- Vitamin B12 — Correct Answer
- Folic acid
- Choline
- Copper
Explanation:
Correct Answer: Vitamin B12
Vitamin B12 (Cyanocobalamin) is the extrinsic Castle factor — so named by W.B. Castle who distinguished it from the gastric (intrinsic) factor. It must be obtained from the diet (hence 'extrinsic') and is found in animal-origin foods (meat, liver, eggs, dairy).
Castle's Factors for RBC Maturation
| Factor | Also Called | Source | Role |
|---|---|---|---|
| Extrinsic Castle factor | Vitamin B12 / Cyanocobalamin | Diet (animal foods) | Required for DNA synthesis and RBC nuclear maturation |
| Intrinsic Castle factor | Gastric intrinsic factor (GIF) | Parietal cells of stomach | Binds to Vit B12 for its absorption in the terminal ileum |
Clinical Application: Pernicious Anemia
When intrinsic factor is absent (e.g., autoimmune destruction of parietal cells or gastrectomy), Vitamin B12 cannot be absorbed → Pernicious anemia develops (Macrocytic, Hyperchromic). Treatment: Intramuscular injection of Vitamin B12.
Why Other Options Are Wrong
- Folic acid → A separate maturation factor; not called Castle's extrinsic factor.
- Choline → A lipotropic factor (prevents fatty liver); not related to RBC maturation.
- Copper → Required for hemoglobin synthesis (iron metabolism) but is not Castle's extrinsic factor.
📚 About this Topic — Animal Physiology
This multiple choice question is from Animal Physiology, Animal Refresher. It has 4 options with a detailed explanation of the correct answer. Practice more MCQs from Animal Physiology to strengthen your preparation.
