**Core Concept**
Fallot's tetralogy is a congenital heart defect characterized by four classic components: ventricular septal defect (VSD), pulmonary stenosis, right ventricular hypertrophy, and an overriding aorta. This complex anomaly leads to reduced blood flow to the lungs, resulting in cyanosis.

**Why the Correct Answer is Right**
In Fallot's tetralogy, the obstruction to blood flow to the lungs (pulmonary stenosis) and the large VSD allow for some right-to-left shunting of blood, bypassing the pulmonary circulation and directly entering the systemic circulation. This shunting results in decreased oxygen saturation and cyanosis, despite the absence of cardiomegaly and congestive heart failure. The right ventricular hypertrophy further contributes to the increased resistance to blood flow through the pulmonary valve.

**Why Each Wrong Option is Incorrect**
**Option A:** Transposition of great arteries (TGA) typically presents with cyanosis and cardiomegaly due to the mixing of oxygenated and deoxygenated blood in the atria, which leads to increased cardiac workload.

**Option C:** Congenital mitral regurgitation usually presents with heart failure symptoms and cardiomegaly due to the volume overload on the left ventricle.

**Option D:** Congenital pulmonary stenosis may present with cyanosis, but it is often associated with cardiomegaly and/or congestive heart failure due to the increased resistance to blood flow through the pulmonary valve, leading to right ventricular hypertrophy and potential right heart failure.

**Clinical Pearl / High-Yield Fact**
Fallot's tetralogy is a classic example of a cyanotic congenital heart defect that may not immediately present with heart failure symptoms, making it essential to consider this diagnosis in patients with unexplained cyanosis.

**✓ Correct Answer: B. Fallot's tetralogy**