## **Core Concept**
Ostium secundum atrial septal defect (ASD) is a type of congenital heart defect characterized by an abnormal opening in the atrial septum, specifically in the region of the fossa ovalis. This defect allows blood to flow between the left and right atria, potentially leading to right-sided volume overload. The clinical presentation and electrocardiographic (ECG) findings are crucial for its diagnosis.

## **Why the Correct Answer is Right**
The correct answer involves identifying false statements about ostium secundum ASD. Typically, ostium secundum ASD is associated with:
- **Fixed splitting of the 2nd heart sound (S2)**: This is a hallmark due to the increased volume in the right heart, which delays the closure of the pulmonary valve relative to the aortic valve, and this delay is not affected by respiration.
- **Right axis deviation (RAD) on ECG**: This is common due to the right ventricular enlargement or hypertrophy secondary to increased volume load.
- **Shunt murmur**: A systolic ejection murmur can be present due to increased flow across the pulmonary valve.

## **Why Each Wrong Option is Incorrect**
- **Option B: Narrow splitting of 2nd heart sound**: This is incorrect because ostium secundum ASD typically presents with **fixed splitting** of the S2, not narrow splitting. Narrow splitting might be seen in conditions with less effect on the right ventricular volume load or in different types of heart defects.
- **Option C: Lt axis deviation in ECG**: This is incorrect because ostium secundum ASD is more commonly associated with **right axis deviation (RAD)**, not left axis deviation. Left axis deviation might be seen in other types of ASDs, like ostium primum ASD.
- **Option a) Fixed splitting of 2nd heart sound**: This statement is true for ostium secundum ASD, making it an incorrect choice as a false statement.
- **Option d) Shunt murmur prominent**: This can be a true statement as a systolic ejection murmur due to increased flow across the pulmonary valve is often present, making it an incorrect choice as a false statement.
- **Option e) Rt axis deviation**: This statement is true for ostium secundum ASD, making it an incorrect choice as a false statement.

## **Clinical Pearl / High-Yield Fact**
A key clinical feature of ostium secundum ASD is the **fixed splitting of the second heart sound**, which does not vary with respiration. This is a critical diagnostic clue. Additionally, ECG findings of **right axis deviation** and right ventricular hypertrophy are common.

## **Correct Answer: B.**