**Core Concept**
Fallot's tetralogy is a congenital heart defect characterized by a combination of four anomalies: ventricular septal defect (VSD), pulmonary stenosis, right ventricular hypertrophy, and an overriding aorta. This condition leads to cyanosis due to the obstruction of blood flow from the right ventricle to the pulmonary artery.

**Why the Correct Answer is Right**
In Fallot's tetralogy, the pulmonary stenosis component obstructs blood flow to the lungs, resulting in a significant left-to-right shunt through the VSD. However, the obstruction prevents deoxygenated blood from passing through the pulmonary artery, leading to increased oxygenated blood being shunted to the systemic circulation, causing cyanosis. This condition does not typically result in cardiomegaly or congestive heart failure due to the presence of a right-to-left shunt.

**Why Each Wrong Option is Incorrect**
**Option A:** Transposition of great arteries (TGA) is characterized by the aorta arising from the right ventricle and the pulmonary artery from the left ventricle, leading to a parallel circulation. While TGA does result in cyanosis, it often presents with cardiomegaly and congestive heart failure due to the mixing of oxygenated and deoxygenated blood in the systemic and pulmonary circulations.

**Option C:** Congenital mitral regurgitation typically presents with symptoms of congestive heart failure, such as dyspnea and fatigue, due to the regurgitation of blood from the left ventricle into the left atrium. Cyanosis may not be a prominent feature of this condition.

**Option D:** Congenital pulmonary stenosis can present with symptoms of cyanosis, but it often does not lead to cardiomegaly or congestive heart failure unless the stenosis is severe and uncorrected.

**Clinical Pearl / High-Yield Fact**
Fallot's tetralogy is often associated with a characteristic "blue lips" or "boot-shaped" heart on chest X-ray, which can be a diagnostic clue for this condition.

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