A 6-year-old boy with Tetralogy of Fallot presents with bluish discoloration of the lips and tongue. On examination, the structure marked **B** (cyanotic tongue) appears distinctly blue, and the buccal mucosa is also discolored. His hemoglobin is 16 g/dL and oxygen saturation by pulse oximetry is 88%. What is the PRIMARY pathophysiological basis for the cyanosis observed in the structure marked **B**?

Explanation

## Why option 1 is correct Central cyanosis, as evidenced by discoloration of the tongue (structure **B**) and buccal mucosa, indicates the presence of deoxygenated hemoglobin exceeding 5 g/dL in the systemic circulation. In Tetralogy of Fallot, a right-to-left cardiac shunt allows deoxygenated blood to bypass the lungs and enter the systemic circulation directly, resulting in central cyanosis affecting the tongue, lips, and oral mucosa. This is the hallmark distinguishing feature of central cyanosis and is the direct cause of the blue discoloration observed in structure **B**. The patient's elevated hemoglobin (16 g/dL) actually facilitates earlier detection of cyanosis because the absolute amount of deoxygenated hemoglobin reaches the 5 g/dL threshold more readily in polycythemia. ## Why each distractor is wrong - **Option 2 (Peripheral perfusion)**: Peripheral cyanosis from sluggish blood flow affects only the extremities (fingers, toes, nail beds) and spares the tongue and buccal mucosa, which remain pink. The involvement of the tongue (**B**) and oral mucosa explicitly indicates central, not peripheral, cyanosis. - **Option 3 (Methemoglobinemia)**: While methemoglobinemia does cause pseudo-cyanosis with a normal or near-normal pO₂, it presents with a characteristic chocolate-brown appearance of blood and does not respond to supplemental oxygen. The clinical context of Tetralogy of Fallot and the patient's normal hemoglobin variant make this unlikely; moreover, methemoglobinemia would require a specific causative agent (dapsone, primaquine, benzocaine, sodium nitrite). - **Option 4 (Severe anemia)**: Severe anemia actually DELAYS or prevents the appearance of cyanosis because the absolute amount of deoxygenated hemoglobin may never reach the 5 g/dL threshold needed for visible cyanosis, even with significant hypoxia. This patient has polycythemia, not anemia, and cyanosis is clinically evident. **High-Yield:** Central cyanosis (tongue + oral mucosa blue) = deoxygenated Hgb >5 g/dL in systemic circulation; peripheral cyanosis (extremities only, tongue pink) = poor peripheral perfusion with normal central oxygenation. [cite: Hutchison's Clinical Methods; Harrison 21e Ch 38]