A 3-year-old boy with Tetralogy of Fallot (TOF) presents with a hypercyanotic spell. Which is the most common site of right ventricular outflow tract obstruction in TOF?

Explanation

## Anatomical Basis of RVOT Obstruction in TOF **Key Point:** Infundibular (subinfundibular/subpulmonary muscular) stenosis is the most common and characteristic site of right ventricular outflow tract (RVOT) obstruction in Tetralogy of Fallot, present in virtually all cases. ### Frequency of RVOT Obstruction Sites in TOF | Site of Obstruction | Frequency | Mechanism | |---|---|---| | **Infundibular stenosis** | Most common (~100%) | Hypertrophy of septoparietal trabeculations and infundibular (conal) septum; the defining lesion of TOF | | Pulmonary valve stenosis | Common (~60–70%) | Thickened, bicuspid or dysplastic valve; usually coexists with infundibular stenosis | | Supravalvular stenosis | Uncommon | Narrowing of main pulmonary artery above the valve | | Subpulmonary stenosis | Rare | Muscular obstruction immediately below the valve | **High-Yield:** According to standard pediatric cardiology texts (Ghai's Textbook of Pediatrics, Nelson's Textbook of Pediatrics, and Robbins Pathologic Basis of Disease), the **infundibular (subvalvular) stenosis** caused by hypertrophy of the infundibular septum and septoparietal trabeculations is the hallmark and most common RVOT obstruction in TOF. Pulmonary valve stenosis frequently coexists but is not the primary or most common isolated lesion. ### Pathophysiology of Cyanosis in TOF 1. Infundibular + pulmonary stenosis → increased RV pressure 2. RV pressure exceeds LV pressure → right-to-left shunt through VSD 3. Deoxygenated blood bypasses lungs → systemic cyanosis 4. **Hypercyanotic (Tet) spells** occur when **infundibular spasm** acutely worsens RVOT obstruction — this dynamic component is unique to infundibular stenosis **Clinical Pearl:** The **dynamic** nature of infundibular stenosis explains hypercyanotic spells triggered by crying, defecation, or exertion (catecholamine surges cause infundibular muscle spasm). Treatment with beta-blockers (propranolol) targets this dynamic infundibular component. Pulmonary valve stenosis is fixed and does not contribute to acute spell generation. **Mnemonic:** In TOF, think **"Infundibulum First"** — the anterior and cephalad deviation of the infundibular (conal) septum is the primary embryological defect that creates both the RVOT obstruction and the perimembranous VSD. *Reference: Ghai Textbook of Pediatrics, 9th ed.; Nelson Textbook of Pediatrics, 21st ed.; Robbins & Cotran Pathologic Basis of Disease, 10th ed.*