A 28-year-old woman is found to have a ventricular septal defect (VSD) on echocardiography during a routine antenatal scan. Which of the following is the most common location of a VSD?

Explanation

## Ventricular Septal Defects: Anatomical Classification and Frequency **Key Point:** Perimembranous (subarterial) VSDs account for approximately 70–80% of all VSDs, making them the most common type. ### Embryological Development of the Ventricular Septum The ventricular septum develops from four components: 1. **Muscular septum** — grows from the apex toward the base 2. **Endocardial cushions** — form the inlet septum 3. **Conus septum** — divides the outflow tract 4. **Membranous septum** — small fibrous area that completes the septum ### VSD Types and Their Frequency | VSD Type | Location | Embryological Defect | Frequency | Associated Lesions | |----------|----------|----------------------|-----------|---------------------| | **Perimembranous (Subarterial)** | Below aortic and pulmonary valves; extends into muscular septum | Inadequate development of membranous septum and adjacent muscular tissue | 70–80% | May extend into inlet or outlet; risk of aortic regurgitation | | **Muscular** | Entirely within muscular septum; may be apical, mid-septal, or anterior | Defective muscular septal development | 15–20% | Usually isolated; may be multiple ("Swiss cheese" septum) | | **Outlet (Infundibular)** | Below pulmonary and aortic valves in outflow region | Defective conus septum development | 5–7% | Associated with overriding aorta, tetralogy of Fallot, transposition | | **Inlet** | Below AV valves; extends into endocardial cushion region | Failure of endocardial cushion fusion | 5–8% | Associated with cleft mitral valve, endocardial cushion defects | **High-Yield:** **Perimembranous VSDs** are the **most common type** (70–80%) and are the **most common congenital heart defect overall** (accounting for ~20% of all CHDs). ### Clinical Pearl **Perimembranous VSDs** have a unique risk: they may be associated with **aortic valve prolapse and aortic regurgitation** because the defect is located immediately below the aortic valve. This complication can develop over time and may require surgical intervention even if the VSD itself is small. ### Mnemonic **"PVM"** — **P**erimembranous is the **V**ery **M**ost common VSD (70–80% of cases). Alternatively: **"Peri-80"** — Perimembranous accounts for ~80% of VSDs. **Warning:** Do not confuse perimembranous with muscular VSDs. Perimembranous defects involve the membranous septum and adjacent muscular tissue, whereas muscular VSDs are entirely within the muscular portion and are more likely to close spontaneously.