A 14-year-old boy presents with acute onset of right flank pain, which developed after he helped his father paint the ceiling of his bedroom. Physical examination demonstrates an area of ecchymosis in the right flank that is tender to palpation. The patient has a lifelong history of easy bruising. His brother shows the same tendency. The serum level of clotting factor VIII is less than 2% of normal. Which of the following is the most likely underlying mechanism for bleeding tendency in this patient?

**Core Concept:** Hemophilia is a rare bleeding disorder caused by deficiency or dysfunction of clotting factors. Factor VIII is a key clotting factor involved in the intrinsic pathway of blood coagulation.

**Why the Correct Answer is Right:** In this case, the patient has a history of easy bruising and a serum level of clotting factor VIII less than 2% of normal. Factor VIII deficiency is the most suitable explanation for his bleeding tendency among the given options. Hemophilia A, also known as classic hemophilia, is associated with a deficiency or dysfunction of clotting factor VIII, leading to impaired blood clot formation and increased bleeding tendency.

**Why Each Wrong Option is Incorrect:**

A. Factor IX deficiency (Hemophilia B) is a less likely cause because it is associated with decreased factor IX levels, not factor VIII.
B. Vitamin K deficiency (Pernicious anemia, warfarin exposure) is not related to easy bruising and factor VIII deficiency, as vitamin K deficiency primarily affects clotting factors II, VII, IX, and X, not factor VIII.
C. Portal vein thrombosis is a condition where thrombosis occurs within the portal vein, affecting liver function and contributing to bleeding tendency. However, this option does not address the factor VIII deficiency and is less specific to the patient's symptoms.
D. Protein C deficiency (Hemophilia C) is a less likely cause for this patient's symptoms since it affects factors V and VIII, but the patient's history of easy bruising is more consistent with factor VIII deficiency.

**Clinical Pearl:** A clinical pearl to remember is that hemophilia A is associated with easy bruising, factor VIII deficiency, and a characteristic clinical presentation: "bruisers" (easy bruising), "bleeders" (excessive bleeding), and "bleeding diathesis" (increased tendency to bleed).

**Correct Answer:** D. Protein C deficiency (Hemophilia C) is the most appropriate answer as it matches the patient's history of easy bruising and factor VIII deficiency. However, the question emphasizes that the patient has a history of easy bruising and factor VIII deficiency, which aligns better with hemophilia A (not C). Therefore, we should focus on hemophilia A as the most likely cause for the patient's symptoms.

✓ Correct Answer: D. Genetic defect involving the factor VIII gene