**Question:** Late onset hemorrhagic disease of newborn is characterized by all of the following features, EXCEPT:

A. Hemophilia
B. Vitamin K deficiency
C. Immune system dysfunction
D. Hemolytic anemia

**Core Concept:** Late-onset hemorrhagic disease of newborn (NHO) is a rare bleeding disorder that occurs in healthy term infants, usually within the first two weeks of life. It is primarily caused by vitamin K deficiency, which is essential for the synthesis of coagulation factors VII, IX, X, and protein C, and protein S.

**Why the Correct Answer is Right:** NHO is characterized by bleeding manifestations due to impaired coagulation factor synthesis. Hemolytic anemia (option D) is a condition characterized by destruction of red blood cells, not a cause of bleeding disorder. Hemophilia (option A) is a genetic disorder affecting the function of coagulation factors, which is different from vitamin K deficiency. Immune system dysfunction (option C) is unrelated to vitamin K deficiency or NHO. Vitamin K deficiency (option B) is the primary cause of NHO, leading to impaired coagulation factor synthesis and subsequent bleeding.

**Why Each Wrong Option is Incorrect:**

A. Hemophilia: This is a genetic disorder affecting the function of coagulation factors, which is different from vitamin K deficiency or NHO.
B. Vitamin K deficiency: NHO is primarily caused by vitamin K deficiency, leading to impaired coagulation factor synthesis and bleeding, not hemolytic anemia.
C. Immune system dysfunction: Immune system dysfunction is unrelated to vitamin K deficiency or NHO, as NHO is caused by vitamin K deficiency affecting coagulation factor synthesis.
D. Hemolytic anemia: Hemolytic anemia is a condition characterized by destruction of red blood cells, unrelated to vitamin K deficiency or NHO. Vitamin K deficiency is the primary cause of NHO, leading to impaired coagulation factor synthesis and subsequent bleeding.

**Clinical Pearl:** Vitamin K deficiency is a crucial factor to consider when assessing neonatal bleeding disorders. Late-onset hemorrhagic disease of newborn (NHO) is a result of vitamin K deficiency, which impairs coagulation factor synthesis and increases the risk of bleeding. A thorough understanding of vitamin K deficiency and its relationship to NHO is essential for accurate diagnosis and management of neonatal bleeding disorders.