All of the following statements regarding the development of the interventricular septum are true EXCEPT:

## Development of the Interventricular Septum ### Embryological Timeline and Components **Key Point:** The interventricular septum develops in two phases: muscular growth (weeks 4–7) followed by membranous closure (weeks 5–8). It is NOT derived from a single source. ### Sequential Formation of the IVS ```mermaid flowchart TD A[Week 4: Myocardial proliferation begins]:::outcome --> B[Muscular IVS grows from apex upward]:::action B --> C[Week 5-7: Muscular septum enlarges]:::action C --> D[Endocardial cushions fuse]:::action D --> E[Membranous IVS closes the gap]:::action E --> F[Week 8: Septation complete]:::outcome G[Neural crest cells contribute to conus septation]:::outcome --> E ``` ### Anatomical Components and Origins | Component | Origin | Timing | Location | | --- | --- | --- | --- | | **Muscular IVS** | Myocardial proliferation from ventricular wall | Week 4–7 | Bulk of septum; grows apex → base | | **Membranous IVS** | Fusion of endocardial cushions + conus septum | Week 5–8 | Small superior portion; closes last | | **Conus septum** | Neural crest cells | Week 5–7 | Contributes to upper membranous septum | ### Why Option 4 is Incorrect **High-Yield:** The interventricular septum is **NOT** derived exclusively from endocardial cushions. It has three sources: 1. **Muscular IVS** — myocardial proliferation (majority) 2. **Membranous IVS** — endocardial cushions + conus septum (neural crest) 3. **Conus septum** — neural crest cells ### Clinical Correlation: VSD Classification **Clinical Pearl:** The location and embryological origin of a VSD determines its classification and clinical significance: | VSD Type | Embryological Defect | Frequency | Prognosis | | --- | --- | --- | --- | | **Muscular** | Failure of muscular septum growth | 20% | Often spontaneous closure | | **Membranous** | Failure of endocardial cushion fusion | 70% | May require surgery; near conduction system | | **Inlet** | Endocardial cushion defect | 5% | Associated with Down syndrome | | **Outlet (Doubly committed)** | Conus septation failure | 5% | Risk of aortic regurgitation | **Warning:** Membranous VSDs are dangerous because they lie adjacent to the AV node and bundle of His; surgical closure carries risk of heart block. ### Mnemonic **"MEM-CONUS"** — **M**uscular (myocardial), **E**ndocardial cushions, **M**embranous; **C**onus septum (neural crest), **O**rigin of upper septum, **N**ot one source, **U**pper membranous, **S**eptation complete.