## Correct Answer: D. Bile duct obstruction Bile duct obstruction causes **vitamin K malabsorption** due to impaired bile salt secretion into the intestine. Vitamin K is a fat-soluble vitamin requiring bile salts for emulsification and absorption in the terminal ileum. Without adequate bile flow, vitamin K cannot be absorbed, leading to deficiency of vitamin K–dependent clotting factors (II, VII, IX, X). Parenteral (intramuscular or intravenous) vitamin K **bypasses the intestinal absorption pathway** and directly replenishes hepatic stores, allowing the liver to synthesize these factors and correct the prolonged PT. This is the classic scenario where parenteral vitamin K is therapeutic. The key discriminator is that the liver is **structurally and functionally intact**—it can synthesize factors if given the cofactor. In bile duct obstruction (whether from stones, strictures, or malignancy), vitamin K repletion will restore coagulation within 12–24 hours. This is a standard Indian clinical presentation, particularly in patients with choledocholithiasis or pancreatic head tumors causing biliary obstruction. ## Why the other options are wrong **A. Hepatitis A** — Hepatitis A causes **hepatocellular injury**, not malabsorption. The liver itself is damaged and cannot synthesize clotting factors even if vitamin K is provided. Parenteral vitamin K will NOT correct the PT because the problem is loss of hepatic synthetic function, not vitamin K deficiency. Hepatitis A is acute and self-limited, but during the acute phase, PT prolongation reflects liver damage, not vitamin K lack. **B. Pernicious anemia** — Pernicious anemia is a **vitamin B12 deficiency** disorder due to lack of intrinsic factor, causing megaloblastic anemia and neurological complications. It has no direct relationship to vitamin K metabolism or coagulation. Parenteral vitamin K is irrelevant to B12 deficiency. This is an NBE distractor exploiting confusion between different parenteral vitamin therapies. **C. Hemophilia B** — Hemophilia B is a **congenital deficiency of factor IX** due to genetic mutation, not vitamin K deficiency. The liver cannot synthesize factor IX because the gene is defective. Parenteral vitamin K will not help because the problem is genetic, not nutritional. This is a trap for students who conflate factor IX deficiency (the stem) with hemophilia B without recognizing the acquired vs. congenital distinction. ## High-Yield Facts - **Vitamin K–dependent factors**: II, VII, IX, X (mnemonic: 2, 7, 9, 10 are even numbers or 'PIVKA' = Proteins Induced by Vitamin K Absence). - **Bile duct obstruction** causes vitamin K malabsorption because bile salts are required for emulsification and absorption of this fat-soluble vitamin in the terminal ileum. - **Parenteral vitamin K** (IM/IV) bypasses intestinal absorption and directly replenishes hepatic stores; effective only if the liver is structurally intact. - **PT corrects within 12–24 hours** of parenteral vitamin K in malabsorption; if PT does not correct, suspect hepatocellular disease or DIC. - **Vitamin K deficiency** causes prolonged PT with normal aPTT (factors II, VII, X are PT-dependent; factor IX is aPTT-dependent, but isolated IX deficiency is rare in acquired disease). ## Mnemonics **PIVKA (Proteins Induced by Vitamin K Absence)** Factors II, VII, IX, X are vitamin K–dependent. When K is absent, these factors are not carboxylated and become non-functional. Remember: **P**rothrombin (II), **I**ntermediate (VII), **V**alue (IX), **K**inase (X)—or simply 2, 7, 9, 10. **When Parenteral Vitamin K Works: LIVER INTACT** Parenteral K helps only if the **liver is structurally sound**. In bile duct obstruction (malabsorption), liver is intact → K works. In hepatitis/cirrhosis (liver damage), liver is damaged → K fails. This is the clinical pearl that discriminates the correct answer. ## NBE Trap NBE pairs "Factor IX deficiency" in the stem with "Hemophilia B" in the options to trap students who assume any factor IX deficiency = hemophilia B, without recognizing that acquired vitamin K deficiency also causes factor IX deficiency. The key is distinguishing **acquired (malabsorption) vs. congenital (genetic)** deficiency. ## Clinical Pearl In Indian clinical practice, a patient with obstructive jaundice (pale stools, dark urine, pruritus) presenting with bleeding manifestations or prolonged PT should immediately raise suspicion for vitamin K deficiency. A single IM dose of vitamin K (10 mg) followed by PT reassessment at 24 hours is diagnostic and therapeutic—if PT corrects, malabsorption was the cause; if it persists, hepatocellular disease is likely. _Reference: Harrison Ch. 139 (Coagulation Disorders); KD Tripathi Ch. 18 (Anticoagulants & Vitamin K); Robbins Ch. 4 (Hemostasis)_
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