Aortic Coarctation

USMLE Step 1 trap: Inverts the BP gradient in coarctation, placing higher pressure in the lower rather than upper extremities. Coarctation causes higher BP in the upper extremities and lower (or absent) BP in the lower extremities due to obstruction distal to the left subclavian artery.

Aortic coarctation is a narrowing of the aorta, classically just distal to the left subclavian artery at the level of the ductus arteriosus, and it is tested on USMLE Step 1 through its BP gradient, CXR findings, and classic associations. Students consistently invert the BP gradient — they assume obstruction raises pressure downstream, but the narrowing backs pressure up into the upper extremities; the lower extremities are supplied through collaterals at reduced pressure. The body compensates by developing collateral circulation through the internal mammary and intercostal arteries — and those collaterals are exactly what the exam loves to probe. Questions often present a vignette with a young woman or child with hypertension and ask you to connect the dots.

What makes this topic tricky is that students routinely invert the BP gradient — they assume the obstruction causes high pressure downstream, when in fact the obstruction backs pressure up into the upper extremities. The lower extremities are perfused through collaterals at reduced pressure. That counterintuitive directionality is exactly where test-writers put the trap. Similarly, students who haven't thought carefully about the anatomy of collateral flow get the CXR finding wrong, misidentifying which ribs show notching.

The associations are the other landmine. Bicuspid aortic valve is present in up to 50–85% of coarctation cases — it's not a coincidence finding, it's the rule. Turner syndrome (45,X) is the classic genetic association. USMLE Step 1 will give you a short-statured female with amenorrhea and hypertension and expect you to connect her karyotype to her cardiac anatomy.

From real student decks

78%have cards covering this topic

65%have mature cards

Common misconceptions

Common mistake

Wrong: Coarctation causes higher BP in the lower extremities than the upper extremities.

Right: Coarctation causes higher BP in the upper extremities and lower (or absent) BP in the lower extremities due to obstruction distal to the left subclavian artery.

The obstruction in coarctation blocks forward flow into the lower aorta, so pressure builds up proximal to the narrowing — meaning in the aortic arch and its branches, which supply the upper extremities. The lower extremities receive blood only through collateral vessels at reduced pressure, so BP there is low, not high. Think of it like pinching a garden hose: pressure rises upstream of the pinch, not downstream.

Common mistake

Wrong: Rib notching in coarctation appears on the first and second ribs.

Right: Rib notching in coarctation appears on ribs 3–8 due to dilated intercostal collateral arteries; the first two ribs are spared because their intercostals arise above the coarctation.

Rib notching is caused by dilated intercostal arteries that serve as collateral channels, and these arteries erode the inferior margins of the ribs over time. The key anatomical detail is that the first and second intercostal arteries arise from the costocervical trunk (a branch of the subclavian), which is above the coarctation — so they don't need to serve as collaterals and don't dilate. Ribs 3–8 use posterior intercostals that arise from the descending aorta below the coarctation, so those are the ones that hypertrophy and cause notching.

Common mistake

Gap: Missing the association of coarctation with bicuspid aortic valve and Turner syndrome

Coarctation is strongly associated with bicuspid aortic valve and Turner syndrome (45,X), and these associations are high-yield for USMLE.

Bicuspid aortic valve and aortic coarctation share a developmental origin — both involve abnormal formation of the left-sided cardiac structures — and they co-occur so frequently that you should assume bicuspid valve until proven otherwise in any coarctation patient. Turner syndrome (45,X) is the classic genetic association; the combination of short stature, webbed neck, primary amenorrhea, and hypertension in a female should immediately prompt you to consider coarctation. Missing these associations on USMLE Step 1 means missing the diagnosis in a classic vignette.

Free Deck audit

See if your Anki deck covers this topic.

Upload your deck →

Guided session

Stuck on this? An AI tutor that probes your understanding.

Start a session →

What the exam tests

Given a patient with hypertension in the arms and weak or absent pulses in the legs, identify the BP gradient pattern and explain why coarctation causes higher pressure above the obstruction and lower pressure below it.
On a chest X-ray description, identify rib notching as a sign of coarctation and know which specific ribs are affected (3–8) and why the first two ribs are spared.
Recognize the syndromic and valvular associations of coarctation — particularly Turner syndrome (45,X) and bicuspid aortic valve — and be able to identify coarctation as the likely diagnosis when these are present in a vignette.

Can you avoid these mistakes?

A 16-year-old female with short stature, webbed neck, and no menstrual periods is found to have BP of 160/90 in her right arm and 90/60 in her right leg. What is the most likely cardiac structural abnormality associated with her underlying diagnosis, and what karyotype do you expect?

On a chest X-ray, a radiologist notes notching along the inferior margins of the posterior ribs. Which ribs show this finding in aortic coarctation, and what is the anatomical reason ribs 1 and 2 are spared?

A patient with coarctation has bounding pulses in the radial artery but weak, delayed pulses in the femoral artery. Explain the hemodynamic mechanism behind this discrepancy — which direction is the BP gradient, and why?

You are told a patient has aortic coarctation. Without any other information, what valvular lesion should you proactively look for on echocardiography, and approximately how common is this association?

Aortic Coarctation

Common misconceptions

What the exam tests

Can you avoid these mistakes?

Related topics