A 14-month-old boy is brought to the clinic by his parents because of a swollen and tender… — USMLE Step 2 CK

A 14-month-old boy is brought to the clinic by his parents because of a swollen and tender right knee that developed yesterday. The patient recently started walking independently. The parents deny any major trauma but note that he frequently bumps into furniture. His temperature is $37.1^\circ\text{C}$ ( $98.8^\circ\text{F}$ ), pulse is $110/\text{min}$ , and respirations are $24/\text{min}$ . Physical examination reveals a moderately swollen, warm right knee with a limited range of motion due to pain. There are also several large ecchymoses on his shins. Laboratory evaluation shows:

Hemoglobin: $11.8 \text{ g/dL}$
Platelet count: $310,000/\text{mm}^3$
Prothrombin time (PT): $12 \text{ seconds}$ (Normal: $11-15 \text{ seconds}$ )
Activated partial thromboplastin time (aPTT): $72 \text{ seconds}$ (Normal: $25-40 \text{ seconds}$ )

Which of the following is the most likely underlying cause of this patient's clinical presentation?

Inherited deficiency of a coagulation cascade proteinAnswer
B
Bacterial invasion of the synovial space
C
Quantitative deficiency of circulating platelets
D
Recurrent physical trauma from delayed gross motor development
E
Benign, self-resolving neonatal vascular reactivity

Answer

The correct answer is an inherited deficiency of a coagulation cascade protein, representing Hemophilia A or B.

The correct answer is an inherited deficiency of a coagulation cascade protein. This patient's presentation is classic for hemophilia (either Hemophilia A, factor VIII deficiency; or Hemophilia B, factor IX deficiency). Hemophilia is an X-linked recessive disorder that frequently becomes symptomatic when an affected infant gains mobility (e.g., crawling or walking). The trauma associated with these normal activities leads to deep tissue bleeding, most classically hemarthrosis (bleeding into the joint spaces), which presents as a warm, swollen, and painful joint. Laboratory evaluation hallmarks include a normal platelet count, a normal prothrombin time (PT), and an isolated prolonged activated partial thromboplastin time (aPTT) reflecting an intrinsic pathway defect.

Step-by-Step Solution

Analyze the clinical presentation.

A 14-month-old boy who just started walking presents with deep tissue bleeding (hemarthrosis of the knee, large ecchymoses) after minimal trauma.

This specific timing (onset of mobility) and bleeding pattern (joints/muscles) points heavily toward a coagulation factor deficiency.

Evaluate the laboratory values.

The patient has a normal platelet count, a normal PT, and an isolated prolongation of the aPTT.

A normal platelet count rules out thrombocytopenia. A normal PT and prolonged aPTT isolate the defect to the intrinsic coagulation pathway.

Synthesize findings to determine the mechanism.

The combination of hemarthrosis in a newly walking male and an isolated prolonged aPTT is classic for Hemophilia A (Factor VIII deficiency) or Hemophilia B (Factor IX deficiency), both of which are inherited coagulation cascade defects.

Matching the clinical syndrome with the specific intrinsic pathway defect solidifies the diagnosis.

Key Concept

Hemophilia typically presents when an affected boy begins walking and experiences minor trauma, leading to deep tissue bleeding (e.g., hemarthrosis). Laboratory evaluation is characterized by an isolated prolonged aPTT with a normal PT and platelet count.

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