Pediatric Murmur Recognition on Board Exams: VSD, ASD, PDA, ToF, AS, and MS

Pediatric murmur questions on COMLEX and USMLE are decided by four features in the stem: where you hear the murmur loudest, where it radiates, when in the cardiac cycle it happens, and the age plus syndrome of the patient. A harsh holosystolic murmur at the lower left sternal border in an otherwise healthy infant is a VSD. A fixed, wide split S2 with a soft systolic murmur is an ASD. A continuous machine-like murmur under the left clavicle is a PDA. A harsh systolic ejection murmur plus cyanotic "tet spells" is tetralogy of Fallot. Lock those four anchors first, then layer the maneuvers and the associations on top.

Most students lose these points not because they don't know the defects, but because they try to match the murmur to a sound they've never actually heard. Boards don't test whether you can auscultate. They test whether you can read a described murmur (location, radiation, timing, quality) and map it onto the right lesion, then predict the syndromic association or the next step. That's a pattern-recognition task, and it's very learnable.

Why do pediatric murmur questions feel harder than they should?

They feel hard because the exam gives you a translated murmur instead of a sound, and most students never built a lookup table for the translation. The fix is to memorize each defect as a bundle of five features (location, radiation, timing/quality, age of presentation, syndrome) so that any two or three features in a stem point to one answer. Once the bundle is automatic, the question collapses fast.

The other reason these feel hard is that the same murmur descriptors get reused across defects, so a single clue rarely discriminates. "Systolic murmur at the left upper sternal border" fits an ASD, pulmonic stenosis, and an innocent flow murmur. You need the second and third feature (a fixed split S2, or a syndrome, or the age) to break the tie. Board writers build the question knowing that one feature is ambiguous on purpose.

Here are the five axes that separate every high-yield pediatric lesion:

1. Location: which listening post is loudest (left lower sternal border, left upper sternal border, apex, right upper sternal border, infraclavicular)
2. Radiation: to the axilla, to the back, to the carotids, or nowhere
3. Timing and quality: holosystolic, systolic ejection, continuous, mid-diastolic rumble, harsh, blowing, machine-like
4. Age of presentation: newborn, first weeks of life, older infant, or not until childhood
5. Syndromic association: Down, DiGeorge, Turner, Williams, congenital rubella, fetal alcohol

Map a stem onto three of those five and the differential usually narrows to one.

VSD: the harsh holosystolic murmur at the lower left sternal border

A ventricular septal defect produces a harsh, holosystolic (pansystolic) murmur heard best at the left lower sternal border, often with a palpable thrill. It's the most common congenital heart defect overall, and it's acyanotic because the shunt is left to right. Counterintuitively for students, a smaller VSD is often louder than a large one, because the higher-velocity jet through a tight hole makes more noise.

The murmur usually isn't present at birth. Pulmonary vascular resistance is still high in the first day or two of life, so the left-to-right pressure gradient (and the murmur) develops over the first few days to weeks as that resistance drops. That timing detail is a favorite board discriminator: a murmur that shows up at the 2-week well-child check fits a VSD better than one screaming on day one.

Key VSD board facts:

• Location: left lower sternal border, harsh holosystolic, often with a thrill
• Shunt: left to right, acyanotic
• Association: most common defect in Down syndrome after AV canal defects, also fetal alcohol syndrome
• Complication: large unrepaired VSDs can reverse into Eisenmenger syndrome (right-to-left shunt, cyanosis, clubbing) if pulmonary hypertension develops
• Course: many small muscular VSDs close spontaneously in childhood

If the stem describes a well-appearing infant with a loud harsh murmur at the lower left sternal border and no cyanosis, VSD is your default answer.

ASD: the fixed, widely split S2

An atrial septal defect classically produces a wide, fixed split S2, meaning the S2 split does not change with inspiration or expiration. The murmur itself is often soft: a systolic ejection murmur at the left upper sternal border from increased flow across the pulmonic valve, sometimes with a mid-diastolic rumble at the lower sternal border from increased flow across the tricuspid valve. The defect is acyanotic with a left-to-right shunt.

The fixed split is the single most tested feature, so understand the mechanism. Normally the split widens with inspiration because increased venous return delays pulmonic valve closure. In an ASD, the atria are functionally connected, so respiratory changes in venous return get equalized between the two sides. The split stays wide and does not vary. That's why "fixed" is the keyword, not just "wide."

ASDs are frequently silent in infancy and get picked up later in childhood or even adulthood. The ostium primum type is associated with Down syndrome. Ostium secundum is the most common type overall. A right-to-left shunt through an ASD can allow a venous clot to reach the systemic circulation, which is the mechanism of a paradoxical embolism, another commonly tested downstream concept.

PDA: the continuous machine-like murmur

A patent ductus arteriosus produces a continuous "machine-like" murmur heard best at the left infraclavicular area, under the left clavicle, spanning both systole and diastole. Because aortic pressure exceeds pulmonary pressure throughout the cardiac cycle, blood shunts left to right continuously, so the murmur never stops at S2 the way a systolic murmur does. It's acyanotic when the shunt stays left to right.

The classic associations are worth memorizing as a pair. Congenital rubella is the syndrome buzzword (often bundled with cataracts and sensorineural deafness). Prematurity is the other high-yield setting, because a preterm infant's ductus is more likely to stay open.

The management contrast is a favorite two-part board trick:

• To close a PDA (the usual goal): indomethacin or another NSAID, which drops prostaglandin and lets the ductus constrict
• To keep a PDA open (when it's the only source of systemic or pulmonary flow, as in a duct-dependent lesion): prostaglandin E1

If a stem gives a cyanotic newborn who is duct-dependent (transposition of the great arteries, for example) and asks how to stabilize before surgery, the answer is prostaglandin E1 to keep the duct open. Same molecule, opposite direction, so read which way the question is pointing.

ToF: the cyanotic tet-spell lesion

Tetralogy of Fallot is the most common cyanotic congenital heart defect presenting after the newborn period, and the murmur you hear is from the pulmonic stenosis, not the VSD. It's a harsh systolic ejection murmur at the left upper sternal border. The four components are pulmonic stenosis, right ventricular hypertrophy, an overriding aorta, and a VSD, and the degree of right ventricular outflow obstruction determines how cyanotic the child is.

The buzzwords are the "boot-shaped heart" on chest X-ray (from RVH) and "tet spells," episodes of acute cyanosis when the child cries, feeds, or exerts. The classic exam detail is the older infant or toddler who squats during a spell. Squatting increases systemic vascular resistance, which decreases the right-to-left shunt across the VSD and pushes more blood through the lungs, relieving the cyanosis. That physiology, not the squatting image alone, is what boards want you to explain.

Tetralogy of Fallot is the cardiac lesion most associated with DiGeorge syndrome (22q11.2 deletion), so a stem pairing conotruncal heart disease with hypocalcemia, an absent thymus, or recurrent infections is pointing you there. It also appears in Down syndrome and fetal alcohol syndrome, but the DiGeorge link is the highest-yield pairing.

AS and MS: the valve lesions that show up in kids

Congenital aortic stenosis and pediatric mitral disease round out the differential, and their listening posts and radiation are the discriminators. Aortic stenosis is a crescendo-decrescendo systolic ejection murmur at the right upper sternal border that radiates to the carotids, often with an ejection click. Mitral stenosis is a low-pitched mid-diastolic rumble at the apex, best heard in the left lateral decubitus position, classically with an opening snap after S2.

Aortic stenosis in the pediatric stem

In children, congenital aortic stenosis is often due to a bicuspid aortic valve, the most common congenital heart malformation overall when you count valve morphology. The murmur is systolic, harsh, and radiates up to the carotids, which is the feature that separates it from most other left-sided murmurs. Turner syndrome is the classic syndromic pairing (along with coarctation of the aorta), so a short-statured girl with a webbed neck and a systolic murmur should trigger both. Supravalvular aortic stenosis is the pairing for Williams syndrome, which also brings the "elfin facies" and infantile hypercalcemia buzzwords.

Mitral stenosis and the apex rumble

Isolated congenital mitral stenosis is uncommon, but the murmur description is high-yield because it contrasts so cleanly with the others: a mid-diastolic rumble at the apex with an opening snap, loudest in the left lateral decubitus position. In the pediatric and young-adult board world, acquired mitral stenosis from rheumatic heart disease is the more testable version, so a stem with a recent untreated strep pharyngitis, migratory arthritis, or chorea followed by a diastolic apical rumble is pointing at rheumatic mitral stenosis.

How do bedside maneuvers change murmurs on boards?

Maneuvers work by changing preload or afterload, and boards test whether you can predict the direction of change. Most murmurs get louder with more blood flowing across the valve, which means most get louder with increased venous return (squatting, leg raise, expiration for left-sided murmurs). The two famous exceptions, hypertrophic cardiomyopathy and mitral valve prolapse, get louder with less preload, so they behave the opposite way.

Use this as your maneuver lookup table:

• Increased venous return / preload (squatting, passive leg raise, lying down): louder VSD, AS, MS, and most flow murmurs; softer HCM and MVP
• Decreased venous return / preload (standing, Valsalva strain phase): softer VSD, AS, MS; louder HCM and MVP
• Increased afterload (handgrip, squatting): louder VSD, mitral regurgitation, aortic regurgitation; softer HCM and AS murmur intensity
• Inspiration: louder right-sided murmurs (tricuspid, pulmonic, and the murmurs of ToF and pulmonic stenosis), by the Carvallo sign

For the pediatric lesions in this article, the two maneuver facts most likely to be tested are the squatting relief of a tet spell (increased afterload cuts the right-to-left shunt) and the fixed split S2 of an ASD that does not vary with respiration at all. If you can reason from preload and afterload rather than memorizing a table blindly, you'll handle the HCM and MVP curveballs too.

Side-by-side comparison table

Drill this table until you can reproduce it from memory. If you can fill in the murmur and the syndrome for each lesion, you can answer most pediatric cardiology board questions in well under a minute.

A note on innocent murmurs, because boards test the flip side too. A Still's murmur (a musical or vibratory systolic murmur at the lower left sternal border in a well child that softens on standing) and a venous hum (a continuous murmur that disappears when you turn the head or lie the child down) are both benign. If the stem stresses that the child is thriving, growing normally, and has no other findings, the exam may want you to reassure rather than work up.

Common board pitfalls and how to avoid them

Pitfall 1: Calling every continuous murmur a PDA. A venous hum is also continuous, but it's benign and it disappears with position change or gentle jugular compression. The PDA machine-like murmur is loudest under the left clavicle and does not vanish when you reposition the child.

Pitfall 2: Assuming the loudest murmur is the most dangerous defect. A small VSD is often louder than a large one because of the high-velocity jet. Loudness tracks turbulence, not severity, so don't let a grade 4 thrill push you toward "large defect" automatically.

Pitfall 3: Forgetting that VSD murmurs are usually absent at birth. High neonatal pulmonary vascular resistance delays the left-to-right gradient. A murmur appearing at the 2-week visit fits a VSD; a murmur present on day one is more likely an obstructive lesion like aortic or pulmonic stenosis.

Pitfall 4: Missing the prostaglandin direction. NSAIDs (indomethacin) close a PDA. Prostaglandin E1 keeps a duct open for a duct-dependent cyanotic newborn. The stem tells you which direction it wants, so read for whether the duct is a problem or a lifeline.

Pitfall 5: Overlooking the syndrome pairing. Hypocalcemia plus a conotruncal defect is DiGeorge and tetralogy of Fallot. A webbed neck and short stature is Turner with coarctation and bicuspid aortic stenosis. Elfin facies with hypercalcemia is Williams with supravalvular aortic stenosis. The syndrome often confirms the murmur you already suspected.

Practice questions

Cover the answer choices, work through the stem, then check yourself against the explanation.

Question 1

A 3-week-old infant is brought to the clinic for a routine well-child visit. The parents report that the baby feeds well and is gaining weight appropriately. The infant is not cyanotic. On auscultation, a harsh, grade 4/6 holosystolic murmur is heard best at the left lower sternal border, with a palpable thrill. The second heart sound is normal. Which of the following is the most likely diagnosis?

A. Atrial septal defect B. Patent ductus arteriosus C. Tetralogy of Fallot D. Ventricular septal defect E. Aortic stenosis

Correct answer: D

A harsh, holosystolic murmur at the left lower sternal border with a palpable thrill in a well-appearing, acyanotic infant is the classic description of a ventricular septal defect. The timing fits as well: the murmur was not necessarily present at birth and becomes audible over the first few weeks as pulmonary vascular resistance falls and the left-to-right gradient develops. An atrial septal defect produces a softer systolic ejection murmur with a fixed split S2, not a harsh holosystolic murmur with a thrill (A is wrong). A patent ductus arteriosus produces a continuous machine-like murmur under the left clavicle (B is wrong). Tetralogy of Fallot is cyanotic and produces a systolic ejection murmur from pulmonic stenosis (C is wrong). Aortic stenosis produces a systolic ejection murmur at the right upper sternal border that radiates to the carotids (E is wrong).

Question 2

A 2-year-old boy has recurrent episodes in which he becomes acutely cyanotic and irritable while crying, then reflexively squats, after which the cyanosis improves. On examination, a harsh systolic ejection murmur is heard at the left upper sternal border. A chest radiograph shows a boot-shaped heart. The child has a history of neonatal hypocalcemia and recurrent infections. Which of the following best explains why squatting relieves his cyanotic episodes?

A. Squatting decreases systemic vascular resistance and increases the left-to-right shunt B. Squatting increases systemic vascular resistance and decreases the right-to-left shunt C. Squatting decreases pulmonary vascular resistance and increases pulmonary blood flow D. Squatting increases venous return and improves right ventricular filling E. Squatting decreases heart rate and improves diastolic coronary filling

Correct answer: B

This child has tetralogy of Fallot, signaled by the tet spells, the harsh systolic ejection murmur from pulmonic stenosis, and the boot-shaped heart, with the neonatal hypocalcemia and recurrent infections pointing to associated DiGeorge syndrome (22q11.2 deletion). During a tet spell, increased right-to-left shunting across the VSD sends deoxygenated blood into the systemic circulation. Squatting compresses the femoral arteries and increases systemic vascular resistance (afterload). Higher afterload raises left-sided pressures, which decreases the right-to-left shunt across the VSD and forces more blood through the right ventricular outflow tract into the lungs, improving oxygenation (B is correct). Decreasing systemic vascular resistance would worsen the right-to-left shunt (A is wrong). Squatting does not primarily act by lowering pulmonary vascular resistance (C is wrong). Increased venous return alone would not preferentially relieve the shunt and can even worsen a spell (D is wrong). The mechanism is not a heart-rate or coronary-filling effect (E is wrong).

Frequently asked questions about pediatric murmurs

What is the single most common congenital heart defect tested on boards?

Ventricular septal defect is the most common congenital heart defect overall, and it's the acyanotic lesion boards return to most often. It presents as a harsh holosystolic murmur at the left lower sternal border, frequently with a palpable thrill, in a well-appearing infant. If you're counting valve morphology separately, bicuspid aortic valve is the single most common congenital cardiac malformation, but for the classic "name the defect from the murmur" question, VSD is the default. Many small muscular VSDs close on their own during childhood.

How do I tell a VSD from an ASD on a board question?

Use the S2 and the murmur quality. A VSD gives a harsh holosystolic murmur at the left lower sternal border, often with a thrill, and a normal S2. An ASD gives a softer systolic ejection murmur at the left upper sternal border plus the hallmark wide, fixed split S2 that does not change with respiration. The fixed split is the discriminator, because it reflects the atria being functionally connected so respiratory swings in venous return get equalized. If the stem stresses a split S2 that stays wide through inspiration and expiration, it's an ASD.

Which congenital heart defects are cyanotic versus acyanotic?

The acyanotic left-to-right shunts are VSD, ASD, and PDA, so a well-appearing, pink infant with a murmur usually has one of those three. The cyanotic lesions classically include the "5 T's": tetralogy of Fallot, transposition of the great arteries, truncus arteriosus, tricuspid atresia, and total anomalous pulmonary venous return. Tetralogy of Fallot is the most common cyanotic defect presenting after the newborn period, while transposition is the most common cyanotic lesion presenting in the first day of life. A large left-to-right shunt can also reverse into a right-to-left shunt (Eisenmenger syndrome) and become cyanotic over time.

What syndromes should I pair with which murmurs?

Down syndrome pairs with AV canal (endocardial cushion) defects and VSDs. DiGeorge syndrome (22q11.2 deletion) pairs with conotruncal defects, most famously tetralogy of Fallot and truncus arteriosus, often alongside hypocalcemia and an absent thymus. Turner syndrome pairs with coarctation of the aorta and bicuspid aortic valve. Williams syndrome pairs with supravalvular aortic stenosis and hypercalcemia. Congenital rubella pairs with PDA. Memorizing these pairings lets the syndrome in the stem confirm the murmur you already suspect.

Why does a smaller VSD sometimes make a louder murmur?

Loudness tracks turbulence, not defect size. A small, restrictive VSD forces blood through a tight opening at high velocity, which generates more turbulent flow and a louder, harsher murmur, sometimes with a palpable thrill. A large VSD has a lower-velocity, less turbulent flow across a bigger hole, so the murmur can be surprisingly soft even though the defect is more hemodynamically significant. This is a favorite board trap, because students instinctively equate a loud murmur with a large or dangerous lesion.

How are pediatric murmurs tested differently on COMLEX versus USMLE?

Both exams test the same core lesions and the same murmur descriptions, so the pattern recognition transfers directly. COMLEX tends to favor the classic vignette pairing (boot-shaped heart plus tet spells for ToF, machine-like murmur for PDA, fixed split S2 for ASD) and may fold in an osteopathic or clinical-management angle. USMLE Step 1 pushes harder on the underlying physiology, such as the preload and afterload logic behind maneuvers, the mechanism of the fixed split, and the shunt reversal in Eisenmenger syndrome. If you understand the physiology well enough to explain why each murmur sounds the way it does, you'll handle either exam's version.

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