Vitamin K (Phytomenadione) | Sources, Storage, Functions & Deficiency | USMLE Step 1 Updated Lecture

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Vitamin K (Phytomenadione) | Sources, Storage, Functions & Deficiency | USMLE Step 1 Updated Lecture

In this updated high-yield USMLE lecture, we take a comprehensive yet simplified look at Vitamin K (Phytomenadione) — an essential fat-soluble vitamin that plays a critical role in coagulation physiology, bone metabolism, and newborn health. Perfect for USMLE Step 1 preparation, this video covers everything from dietary sources and absorption to its biochemical functions, storage, and clinical manifestations of deficiency, all tied to high-yield NBME-style scenarios. 💉🧠

Vitamin K exists in two major forms:
– Phylloquinone (K1) – found in green leafy vegetables like spinach, kale, and broccoli
– Menaquinone (K2) – synthesized by gut microbiota in the colon

Vitamin K is absorbed in the small intestine along with dietary fat and requires bile salts for proper absorption. It is stored in small quantities in the liver, which makes deficiency develop rapidly, especially in individuals with fat malabsorption syndromes, prolonged antibiotic use, or neonates without supplementation.

🩸 Physiological Functions:
Vitamin K is a cofactor for the γ-glutamyl carboxylase enzyme that activates several vitamin K–dependent clotting factors:
– Factors II (prothrombin), VII, IX, and X
– Anticoagulant proteins C and S
This carboxylation step allows these proteins to bind calcium and function properly in the coagulation cascade.

⚠️ Deficiency States – High-Yield Points:
– Newborns are at high risk due to low placental transfer, sterile gut, and low liver stores
– Presents as neonatal hemorrhagic disease — bleeding from umbilical stump, GI tract, or intracranial sites
– Adults with prolonged broad-spectrum antibiotic use, cholestasis, or fat malabsorption (e.g., cystic fibrosis, celiac disease, biliary obstruction) may also develop deficiency
– Labs show: ↑ PT/INR, normal bleeding time, normal platelet count

🩺 Clinical Uses:
– Vitamin K1 injection is given routinely at birth to prevent hemorrhagic disease
– Also used to reverse warfarin overdose

This lecture also integrates key comparisons with other fat-soluble vitamins, highlights differences between intrinsic vs acquired causes, and reinforces clinical clues that commonly appear in Step 1 questions.


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