A 4-month-old male infant presents with recurrent cyanotic spells triggered by feeding and crying. On examination, he has central cyanosis, clubbing, a single S2, and a harsh systolic ejection murmur at the left upper sternal border. Chest radiograph shows a boot-shaped heart with decreased pulmonary vascularity. Echocardiography confirms tetralogy of Fallot with all four anatomic lesions. The structure marked **D** (RV hypertrophy) in the diagram develops as a secondary consequence of which primary pathophysiologic mechanism?

Question

Accepted Answer

B. Right-to-left shunting across the VSD due to RVOT obstruction, forcing the RV to generate higher pressures to overcome the outflow resistance. ## Why option 1 is correct

The structure marked **D** (RV hypertrophy) develops as a direct consequence of the RVOT obstruction (infundibular stenosis ± pulmonary valve stenosis) that forces the right ventricle to generate suprasystemic pressures to eject blood across the narrowed outflow tract. This chronic pressure overload triggers concentric hypertrophy of the RV wall. The degree of RV hypertrophy is proportional to the severity of RVOT obstruction. According to Nadas Pediatric Cardiology and AHA guidelines, RV hypertrophy is the fourth and final anatomic lesion in TOF, arising embryologically from the same anterocephalad deviation of the outlet septum that causes the other three lesions, but functionally it represents the RV's adaptive response to chronic outflow obstruction. The right-to-left shunt across the VSD is a consequence of the RVOT obstruction creating a pressure gradient that favors shunting of deoxygenated blood directly into the aorta, bypassing the lungs—this is the mechanism of cyanosis in TOF.

## Why each distractor is wrong

- **Option 2**: In TOF, the shunt is predominantly right-to-left (not left-to-right) because RVOT obstruction creates higher RV pressures than LV pressures at the VSD level. Left-to-right shunting would cause volume overload and eccentric hypertrophy, not the pressure-overload concentric hypertrophy seen in TOF.
- **Option 3**: Pulmonary regurgitation is a long-term complication that develops *after* surgical repair (due to transannular patching), not a primary mechanism of RV hypertrophy in uncorrected TOF. In the acute/unoperated phase, pulmonary stenosis (not regurgitation) is the primary lesion.
- **Option 4**: Aortic override is one of the four anatomic lesions and contributes to the right-to-left shunt, but it does not directly cause RV hypertrophy. The hypertrophy results from pressure overload due to RVOT obstruction, not from the anatomic position of the aorta.

**High-Yield:** RV hypertrophy in TOF is a pressure-overload phenomenon driven by RVOT obstruction; the degree of hypertrophy correlates with obstruction severity and is the fourth lesion arising from a single embryologic defect (anterocephalad outlet septum deviation).

[cite: Nadas Pediatric Cardiology; AHA Adult Congenital Heart Disease Guidelines]

Answer

A. Left-to-right shunting across the VSD, causing volume overload and compensatory RV hypertrophy

Answer

C. Aortic override causing preferential streaming of deoxygenated blood into the systemic circulation

Answer

D. Pulmonary regurgitation from a dysplastic pulmonary valve, leading to chronic RV volume loading

Explanation

Why option 1 is correct

Why each distractor is wrong

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