A 5-year-old boy presents with recurrent hemahroses and intramuscular hematomas. Laboratory tests reveal normal bleeding time, platelet count, and PT, but the PTT is prolonged. This boy’s condition most likely results from an abnormality involving
**Core Concept**
The underlying principle being tested is the understanding of coagulation disorders, specifically the intrinsic and extrinsic pathways of blood coagulation. This involves knowledge of **coagulation factors** and their role in forming a blood clot.
**Why the Correct Answer is Right**
Given the prolonged **PTT (Partial Thromboplastin Time)** with normal **PT (Prothrombin Time)**, **bleeding time**, and **platelet count**, the condition suggests a defect in the intrinsic pathway of coagulation. This pathway involves factors VIII, IX, XI, and XII. A deficiency in any of these factors, most commonly factor VIII (as in **Hemophilia A**) or factor IX (as in **Hemophilia B**), leads to recurrent **hemarthroses** (bleeding into joints) and **intramuscular hematomas**.
**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it doesn't directly relate to the intrinsic pathway defect suggested by the prolonged PTT.
**Option B:** Incorrect as it's not specifically related to the coagulation factor deficiency implied by the clinical presentation and lab results.
**Option C:** Incorrect because, although it might be involved in coagulation, it doesn't directly explain the prolonged PTT and the clinical symptoms described.
**Option D:** Incorrect as it is not directly related to the intrinsic coagulation pathway defect indicated by the patient's symptoms and lab findings.
**Clinical Pearl / High-Yield Fact**
A key point to remember is that **Hemophilia A and B** present with similar clinical symptoms (e.g., hemarthroses, intramuscular hematomas) but are differentiated by the specific factor deficiency (VIII for Hemophilia A, IX for Hemophilia B).
**Correct Answer:** D. Factor VIII