Blood Flow Dynamics in VSD Steps:

• VSD creates a hole between left and right ventricles, allowing blood to shunt based on pressure differences.
• Left ventricle pressure exceeds right ventricle pressure, causing left-to-right shunt of oxygenated blood.
• This mixes oxygen-rich blood into the right ventricle, increasing pulmonary blood flow.
• Right-to-left shunt does not occur initially, preventing early cyanosis.

Why D is correct:

• D (2 and 3 only) aligns with VSD physiology: oxygen-rich blood mixes with oxygen-poor blood (2), and extra blood increases lung flow (3), per pressure gradient laws in congenital heart defects.

Why the others are wrong:

• A includes 1 (right-to-left shunt), which is incorrect as flow is left-to-right.
• B includes 1 but excludes 3, missing the pulmonary overcirculation effect.
• C excludes 2, ignoring the mixing of oxygenated blood into the right side.

Final answer: D