Which of the following is true about vitamin K?

Correct Answer: D. Prolonged use of antibiotics leads to vitamin K deficiency

Vitamin K exists in two main forms: phylloquinone (K1) from dietary sources (green leafy vegetables, cruciferous vegetables) and menaquinone (K2) synthesized by colonic bacteria. The critical point is that approximately 50% of the body's vitamin K requirement comes from bacterial synthesis in the colon, not dietary sources alone. Prolonged antibiotic use—particularly broad-spectrum agents like fluoroquinolones, cephalosporins, and aminoglycosides commonly used in Indian clinical practice—disrupts the normal colonic microbiota. This eliminates the bacterial production of menaquinone, leading to vitamin K deficiency despite adequate dietary intake. This deficiency impairs the synthesis of vitamin K-dependent clotting factors (II, VII, IX, X) and proteins C and S, resulting in a prolonged PT/INR and bleeding tendency. This is a well-recognized complication in Indian hospitals, particularly in patients on prolonged antibiotic courses for infections like tuberculosis (TB) or nosocomial infections. The deficiency manifests within 1–2 weeks of starting antibiotics in susceptible individuals.

Why the other options are wrong

A. Menaquinone is the dietary source of vitamin K found in green vegetables. — This is wrong because phylloquinone (K1), not menaquinone, is the dietary form found in green vegetables. Menaquinone (K2) is synthesized by colonic bacteria and is not a dietary source. This is a classic NBE trap that confuses the two forms and their origins—students who memorize 'K2 from bacteria' may reverse it and pick this option. B. A deficiency of vitamin K leads to hypercoagulability — This is wrong because vitamin K deficiency causes hypocoagulability (bleeding tendency), not hypercoagulability. Deficiency impairs synthesis of clotting factors II, VII, IX, X and proteins C and S, leading to prolonged PT and increased bleeding risk. Hypercoagulability occurs with vitamin K excess or in thrombophilic states—this is a direct reversal trap. C. It is a water-soluble vitamin — This is wrong because vitamin K is fat-soluble (along with vitamins A, D, E), requiring dietary fat for absorption in the small intestine. Water-soluble vitamins (B-complex, C) are not stored in the body and are excreted in urine. This tests basic vitamin classification and is a straightforward distractor for students who confuse vitamin categories.

High-Yield Facts

Mnemonics

K1 vs K2 Origin K1 = Kale (dietary), K2 = Kolon bacteria (synthesis). K1 is phylloquinone from green vegetables; K2 is menaquinone from gut bacteria. Use when distinguishing dietary vs. bacterial sources. Vitamin K-Dependent Factors 2, 7, 9, 10 — the four clotting factors requiring vitamin K for carboxylation (also proteins C and S). Deficiency prolongs PT selectively. Mnemonic: 'Factors 2, 7, 9, 10 need K to go.'

NBE Trap

NBE pairs 'menaquinone' with 'dietary source' to trap students who know K2 is bacterial but reverse the association. The trap exploits confusion between K1 (phylloquinone, dietary) and K2 (menaquinone, bacterial)—a common memorization error in Indian coaching.

Clinical Pearl

In Indian hospitals, vitamin K deficiency from prolonged antibiotics is common in TB patients on anti-TB drugs combined with broad-spectrum cover, or in septic patients. Always check PT/INR in patients on antibiotics >2 weeks and supplement vitamin K1 10 mg IV/IM if prolonged—dietary sources alone cannot compensate for lost bacterial synthesis.

_Reference: Harper Biochemistry Ch. 50 (Vitamins); KD Tripathi Pharmacology Ch. 57 (Anticoagulants & Vitamin K)_