Petechial hemorrhages were noticed on the upper and lower extremities of a 5-day-old infant. Hemorrhagic disease of the neonate was most likely caused by a deficiency of which of the following vitamins?
**Core Concept**
Vitamin K-dependent clotting factors play a crucial role in the coagulation cascade, particularly in the activation of prothrombin and factors II, VII, IX, and X. A deficiency of this vitamin can lead to impaired clotting, resulting in bleeding manifestations such as petechiae in neonates.
**Why the Correct Answer is Right**
The most likely cause of hemorrhagic disease of the neonate is a deficiency of vitamin K. This is because vitamin K is essential for the gamma-carboxylation of clotting factors II, VII, IX, and X, which are produced in the liver. Without sufficient vitamin K, these clotting factors remain uncarboxylated and cannot bind to phospholipid surfaces, thereby preventing the proper activation of the coagulation cascade. This leads to a prothrombin time (PT) prolongation and an increased risk of bleeding.
**Why Each Wrong Option is Incorrect**
* **Option A:** While vitamin C is essential for collagen synthesis and iron absorption, it is not directly involved in the coagulation cascade or the prevention of hemorrhagic disease of the neonate.
* **Option B:** Vitamin B12 is crucial for the synthesis of DNA, fatty acid metabolism, and the maintenance of the nervous system. However, it is not directly involved in the coagulation cascade.
* **Option D:** Folate (vitamin B9) is essential for DNA synthesis and repair, as well as the metabolism of homocysteine. While folate deficiency can lead to megaloblastic anemia, it is not directly related to hemorrhagic disease of the neonate.
**Clinical Pearl / High-Yield Fact**
Vitamin K deficiency can be prevented by administering vitamin K to newborns shortly after birth, typically within the first hour of life. This practice has significantly reduced the incidence of hemorrhagic disease of the neonate.
**Correct Answer:** C. Vitamin K