**Core Concept:**
Neonatal bleeding can be due to various causes including hemolytic disease of the newborn, platelet disorders, coagulation defects, or infections. Vaginal bleeding in neonates is a concerning sign that requires immediate evaluation and management.

**Why the Correct Answer is Right:**
In this scenario, the correct answer is **D:** Administering vitamin K. Vitamin K is essential for the synthesis of clotting factors II, VII, IX, X, protein C, and protein S. In neonates, vitamin K deficiency can lead to neonatal jaundice and increased bleeding tendency. Administering vitamin K helps replenish the deficient clotting factors and stops the bleeding.

**Why Each Wrong Option is Incorrect:**
A. Vitamin K deficiency is unlikely the sole cause of vaginal bleeding in neonates, as it usually presents with more severe bleeding manifestations.
B. Thrombocytopenia (low platelet count) is a different cause of bleeding and would present with petechiae (small, red spots) rather than vaginal bleeding.
C. Coagulation defects (e.g., hemophilia) would present with bleeding manifestations in various body systems, not just vaginal bleeding.

**Clinical Pearl:**
A neonate presenting with vaginal bleeding should be investigated for underlying causes such as hemolytic disease of the newborn, platelet disorders, and coagulation defects. In the absence of a clear cause, vitamin K deficiency should be considered, and vitamin K supplementation can help stop the bleeding.

**Correct Answer:** D. Vitamin K administration