Failure of the interventricular septum to fuse during cardiac development results in which of the following defects?

Explanation

## Embryological Basis of Ventricular Septal Defect (VSD) **Key Point:** The interventricular septum develops from two sources — the **muscular (trabecular) septum** and the **membranous septum** — and fusion defects lead to VSDs, the most common congenital heart defect. ### Development of the Interventricular Septum The interventricular septum forms through the fusion of: 1. **Muscular (trabecular) septum** — grows upward from the apex of the ventricle (weeks 4–7) 2. **Membranous septum** — derived from the endocardial cushions and truncus arteriosus (weeks 5–7) 3. **Conal septum** — derived from neural crest cells; separates the outflow tracts ### Classification of VSDs by Location | VSD Type | Embryological Origin | Frequency | Clinical Significance | | --- | --- | --- | --- | | **Membranous** | Endocardial cushion/truncus defect | 70–80% | Often closes spontaneously; risk of aortic regurgitation | | **Muscular** | Failure of muscular septum growth | 15–20% | May close spontaneously; multiple defects possible | | **Outlet (Conal)** | Conal septum maldevelopment | 5–10% | Associated with TOF, TGA; high risk of aortic regurgitation | | **Inlet** | Endocardial cushion defect | 5% | Associated with endocardial cushion defects; AV valve involvement | **High-Yield:** **Membranous VSDs** account for ~75% of all VSDs and are the most clinically important. They often have spontaneous closure rates of 25–50% by age 5. **Mnemonic:** **MVOC** — **M**embranous (most common), **V**entricular (muscular), **O**utlet (conal), **I**nlet (endocardial cushion) — represents the four types of VSDs. ### Embryological Timing ```mermaid flowchart TD A[Week 4: Muscular septum begins to grow]:::outcome --> B[Week 5-6: Endocardial cushions fuse] B --> C[Week 5-7: Membranous septum forms] C --> D[Week 6-7: Conal septum divides outflow tract] D --> E{All septa fused?}:::decision E -->|Yes| F[Normal ventricular septation]:::outcome E -->|No| G[Ventricular septal defect]:::urgent G --> H[Left-to-right shunt at ventricular level] H --> I[Pulmonary overcirculation]:::outcome ``` **Clinical Pearl:** The **aortic valve** is in close proximity to the membranous septum. Membranous VSDs can lead to **aortic regurgitation** due to prolapse of the right coronary cusp into the defect — this is a critical long-term complication requiring monitoring. ### Why This Is NOT Other Defects - **ASD** — results from defects in the **atrial septum** (ostium secundum, ostium primum, sinus venosus), not the ventricular septum - **PFO** — results from failure of the **foramen ovale to close** postnatally; it is a normal fetal structure that persists - **TOF** — results from **anterior displacement of the conal septum**, leading to a VSD plus right ventricular outflow obstruction (pulmonary stenosis), right ventricular hypertrophy, and overriding aorta