Today’s USMLE® Step 1 question of the day features a 23-year-old woman who wants to know more about newborn’s need for Vitamin K. What’s the answer to her question? Let’s find out!
A 23-year-old primigravid woman who is planning a home birth asks whether her newborn will need vitamin K administration after delivery. The physician explains that newborns are at increased risk for vitamin K deficiency bleeding (VKDB) without prophylaxis.
Which of the following best explains why neonates require Vitamin K administration?
A. Markedly increased vitamin K requirements during the neonatal period
B. Inability of neonates to absorb vitamin K present in breast milk
C. Inability to store vitamin K due to low body fat content
D. Inability to recycle vitamin K after it is used in coagulation reactions
E. Insufficient intestinal colonization by vitamin K–producing bacteria
Scroll down for the correct answer!
The correct answer to today’s USMLE® Step 1 Question is…
E. Insufficient intestinal colonization by vitamin K–producing bacteria
Correct: See Main Explanation.
Incorrect Answer Explanations
A. Markedly increased vitamin K requirements during the neonatal period
Incorrect: Neonatal vitamin K deficiency is primarily caused by decreased vitamin K availability rather than increased physiologic demand.
B. Inability of neonates to absorb vitamin K present in breast milk
Incorrect: Neonates are capable of absorbing vitamin K. However, breast milk contains relatively low amounts of vitamin K, which contributes to low neonatal vitamin K stores.
C. Inability to store vitamin K due to low body fat content
Incorrect: Neonates are able to store vitamin K. The deficiency primarily results from low endogenous and exogenous vitamin K supply.
D. Inability to recycle vitamin K after it is used in coagulation reactions
Incorrect: Neonates possess functional vitamin K epoxide reductase, the enzyme responsible for recycling vitamin K. Although enzyme activity may be somewhat reduced compared with adults, recycling still occurs.
Main Explanation
Vitamin K is a fat-soluble vitamin required for γ-carboxylation of clotting factors II, VII, IX, and X, as well as proteins C and S. This modification activates clotting factors involved in secondary hemostasis and directly contributes to coagulation. Therefore, a deficiency of vitamin K in a neonate can lead to serious bleeding.
For most adults, vitamin K is easily obtained through dietary sources. Vitamin K is also produced endogenously by vitamin K–producing intestinal bacteria. However, neonates are at risk for vitamin K deficiency because both endogenous and exogenous sources of vitamin K are limited after birth. Endogenous vitamin K production is low because the neonatal intestine has not yet been sufficiently colonized by vitamin K–producing bacteria. Exogenous vitamin K supply is also limited due to low placental transfer before birth and the relatively low concentration of vitamin K in breast milk.
Because of these factors, newborns routinely receive intramuscular vitamin K prophylaxis shortly after birth to prevent vitamin K deficiency bleeding (VKDB), which may present with gastrointestinal bleeding, mucosal bleeding, or intracranial hemorrhage.
Major Takeaway
Neonatal vitamin K deficiency is due to decreased endogenous sources (insufficient intestinal colonization by vitamin K–producing bacteria) and decreased exogenous sources (limited placental transfer and low vitamin K concentration in breast milk). Newborns routinely receive vitamin K prophylaxis to prevent vitamin K deficiency bleeding (VKDB).
Want to learn more about this topic?
Watch this Osmosis video: Role of Vitamin K in coagulation
References
- Hand, I., Noble, L., Abrams, S. A., COMMITTEE ON FETUS AND NEWBORN, SECTION ON BREASTFEEDING, & COMMITTEE ON NUTRITION. (2022). Vitamin K and the newborn infant. Pediatrics, 149(3), e2021056036. https://doi.org/10.1542/peds.2021-056036
- Shearer, M. J. (2018). Vitamin K deficiency bleeding (VKDB) in early infancy. Blood Reviews, 32(3), 225–234. https://doi.org/10.1016/j.blre.2017.11.001
- Lippi, G., & Franchini, M. (2011). Vitamin K in neonates: facts and myths. Blood transfusion = Trasfusione del sangue, 9(1), 4–9. https://doi.org/10.2450/2010.0034-10
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