Collateral circulation US Medical PG Practice Questions and MCQs
Practice US Medical PG questions for Collateral circulation. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
Collateral circulation US Medical PG Question 1: A 61-year-old man is brought to the emergency room with slurred speech. According to the patient's wife, they were watching a movie together when he developed a minor headache. He soon developed difficulty speaking in complete sentences, at which point she decided to take him to the emergency room. His past medical history is notable for hypertension and hyperlipidemia. He takes aspirin, lisinopril, rosuvastatin. The patient is a retired lawyer. He has a 25-pack-year smoking history and drinks 4-5 beers per day. His father died of a myocardial infarction, and his mother died of breast cancer. His temperature is 98.6°F (37°C), blood pressure is 143/81 mmHg, pulse is 88/min, and respirations are 21/min. On exam, he can understand everything that is being said to him and is able to repeat statements without difficulty. However, when asked to speak freely, he hesitates with every word and takes 30 seconds to finish a short sentence. This patient most likely has an infarct in which of the following vascular distributions?
- A. Proximal middle cerebral artery
- B. Inferior division of the middle cerebral artery
- C. Middle cerebral artery and posterior cerebral artery watershed area
- D. Superior division of the middle cerebral artery (Correct Answer)
- E. Anterior cerebral artery and middle cerebral artery watershed area
Collateral circulation Explanation: ***Superior division of the middle cerebral artery***
- The patient's inability to speak spontaneously coupled with intact comprehension and repetition is characteristic of **Broca's aphasia**, which results from damage to **Broca's area** in the dominant frontal lobe.
- Broca's area is supplied by the **superior division of the middle cerebral artery (MCA)**.
*Proximal middle cerebral artery*
- An infarct in the proximal MCA, or the main stem, would typically lead to global aphasia if the dominant hemisphere is affected, characterized by **severe deficits in comprehension, repetition, and speech production**.
- This presentation does not align with the patient's ability to understand and repeat statements.
*Inferior division of the middle cerebral artery*
- The inferior division of the MCA supplies Wernicke's area in the dominant hemisphere.
- Damage here causes **Wernicke's aphasia**, characterized by **fluent but nonsensical speech** with **impaired comprehension** and **repetition**, which is contrary to the patient's symptoms.
*Middle cerebral artery and posterior cerebral artery watershed area*
- Watershed infarcts, especially between the MCA and posterior cerebral artery (PCA), can cause **transcortical sensory aphasia** if in the dominant hemisphere.
- This type of aphasia involves impaired comprehension but **intact repetition**, which differs from Broca's aphasia where spontaneous speech is the main deficit.
*Anterior cerebral artery and middle cerebral artery watershed area*
- Infarcts in the watershed area between the anterior cerebral artery (ACA) and MCA can result in **transcortical motor aphasia** if in the dominant hemisphere.
- This condition presents with **non-fluent speech** and **intact repetition**, similar to Broca's aphasia, but Broca's area itself is located specifically within the MCA superior division territory.
Collateral circulation US Medical PG Question 2: A 54-year-old man was brought to the emergency room due to acute onset of slurred speech while at work, after which he lost consciousness. The patient's wife says this occurred approximately 30 minutes ago. Past medical history is significant for poorly controlled hypertension and type 2 diabetes mellitus. His blood pressure is 90/50 mm Hg, respiratory rate is 12/min, and heart rate is 48/min. The patient passes away shortly after arriving at the hospital. At autopsy, bilateral wedge-shaped strips of necrosis are seen in this patient's brain in the medial temporal lobe structures. Which of the following is the most likely location of these necrotic cells?
- A. Frontal lobe
- B. Hippocampus (Correct Answer)
- C. Cortex or cerebral hemisphere
- D. Substantia nigra
- E. Caudate nucleus
Collateral circulation Explanation: ***Hippocampus***
- The description of wedge-shaped necrosis just below the **medial temporal lobes** points directly to the **hippocampus**, which is highly susceptible to **ischemic injury**.
- The patient's **hypotension** and subsequent death suggest an event causing global cerebral hypoperfusion, making the hippocampus vulnerable due to its high metabolic demand and sensitivity to oxygen deprivation.
*Frontal lobe*
- While the frontal lobe can be affected by ischemia, its location is not consistent with "just below the **medial temporal lobes**" and the necrotic pattern described is more characteristic of specific vulnerable regions.
- Involvement of the frontal lobe would typically present with different focal neurological deficits depending on the specific area affected, such as motor weakness or personality changes.
*Cortex or cerebral hemisphere*
- **Wedge-shaped necrosis** is a pattern often seen in watershed areas or specific vulnerable regions, not a general description for global cortical ischemia.
- While the cortex is broadly affected by global ischemia, the specific localization described is much more precise than "cortex or cerebral hemisphere."
*Substantia nigra*
- The substantia nigra is located in the **midbrain** and is primarily involved in motor control, not typically implicated in the described **wedge-shaped necrosis** pattern associated with global ischemia below the medial temporal lobes.
- Damage to the substantia nigra is more commonly associated with conditions like **Parkinson's disease**.
*Caudate nucleus*
- The caudate nucleus is part of the **basal ganglia**, located deep within the cerebral hemispheres, and is not described as being "just below the **medial temporal lobes**."
- Ischemic damage to the caudate nucleus would cause different symptoms and typically not present with the specific necrotizing pattern described.
Collateral circulation US Medical PG Question 3: A 75-year-old man is brought to the emergency room after being found unresponsive in his home. His medical history is unknown. On physical examination he does not demonstrate any spontaneous movement of his extremities and is unable to respond to voice or painful stimuli. You notice that he is able blink and move his eyes in the vertical plane. Based on these physical exam findings, you expect that magnetic resonance angiogram will most likely reveal an occlusion in which of the following vessels?
- A. Basilar artery (Correct Answer)
- B. Anterior cerebral artery
- C. Anterior spinal artery
- D. Posterior cerebral artery
- E. Anterior inferior cerebellar artery
Collateral circulation Explanation: ***Basilar artery***
- The patient's presentation with **quadriplegia**, inability to respond to stimuli, and preserved **vertical eye movements** and blinking is characteristic of **locked-in syndrome**.
- **Locked-in syndrome** is most commonly caused by an **occlusion of the basilar artery**, leading to infarction of the ventral pons while sparing the tegmentum.
*Anterior cerebral artery*
- Occlusion of the **anterior cerebral artery** typically causes **contralateral leg weakness** and sensory loss, and sometimes **abulia** or urinary incontinence.
- It does not explain the widespread motor paralysis affecting all four limbs or the preservation of vertical eye movements in an otherwise unresponsive patient.
*Anterior spinal artery*
- Occlusion of the **anterior spinal artery** causes an **anterior spinal cord syndrome**, characterized by bilateral weakness and loss of pain/temperature sensation below the lesion, with preserved proprioception and vibration sense.
- This presentation does not include the characteristic signs of **brainstem dysfunction** seen in locked-in syndrome.
*Posterior cerebral artery*
- An occlusion in the **posterior cerebral artery** typically leads to **contralateral homonymous hemianopia**, and depending on the branch occluded, can also cause memory deficits or thalamic pain.
- It does not account for the extensive motor paralysis or the pattern of preserved vertical eye movements seen in locked-in syndrome.
*Anterior inferior cerebellar artery*
- Occlusion of the **anterior inferior cerebellar artery (AICA)** typically results in **lateral pontine syndrome**, which includes ipsilateral facial weakness, hearing loss, cerebellar ataxia, and contralateral loss of pain and temperature sensation.
- While it affects the pons, AICA occlusion does not typically cause the complete quadriplegia and preservation of vertical eye movements characteristic of locked-in syndrome.
Collateral circulation US Medical PG Question 4: A 53-year-old man with a past medical history significant for hyperlipidemia, hypertension, and hyperhomocysteinemia presents to the emergency department complaining of 10/10 crushing, left-sided chest pain radiating down his left arm and up his neck into the left side of his jaw. His ECG shows ST-segment elevation in leads V2-V4. He is taken to the cardiac catheterization laboratory for successful balloon angioplasty and stenting of a complete blockage in his left anterior descending coronary artery. Echocardiogram the following day shows decreased left ventricular function and regional wall motion abnormalities. A follow-up echocardiogram 14 days later shows a normal ejection fraction and no regional wall motion abnormalities. This post-infarct course illustrates which of the following concepts?
- A. Coronary collateral circulation
- B. Ventricular remodeling
- C. Myocardial hibernation
- D. Myocardial stunning (Correct Answer)
- E. Reperfusion injury
Collateral circulation Explanation: ***Myocardial stunning***
- This refers to a temporary **post-ischemic contractile dysfunction** that persists even after blood flow has been restored following an acute ischemic event.
- The return to normal left ventricular function and absence of regional wall motion abnormalities after successful reperfusion indicates that the initial dysfunction was transient and not due to permanent myocardial damage.
- Classic timeframe: recovery occurs over **days to weeks** after reperfusion, as seen in this patient (14 days).
*Coronary collateral circulation*
- This involves the development of alternative pathways for blood supply to the myocardium when the primary coronary arteries are occluded.
- While it can mitigate the extent of myocardial injury, it generally doesn't explain the reversal of severe regional wall motion abnormalities and low ejection fraction to normal in such a short period after a complete blockage.
*Ventricular remodeling*
- This refers to changes in the **size, shape, and function of the ventricles** in response to myocardial injury or chronic pressure/volume overload, often leading to progressive heart failure.
- It typically involves *persistent* and *often detrimental* changes, which is contrary to the improvement seen in this patient's echocardiogram.
*Myocardial hibernation*
- This is a state of **persistently impaired myocardial function at rest** due to **chronic inadequate blood flow** that can improve with revascularization.
- Hibernation requires **pre-existing chronic ischemia** with baseline dysfunction prior to intervention, not an acute complete occlusion presenting as STEMI.
- This patient had an **acute presentation** with complete blockage and no history suggesting chronic stable ischemia, making stunning (not hibernation) the correct answer.
*Reperfusion injury*
- This is damage to the myocardial tissue that occurs **after blood flow is restored** to an ischemic area, often involving oxidative stress and inflammation.
- While it can worsen myocardial function, it is a complication of reperfusion that causes *additional damage*, not a phenomenon that explains the *recovery* of cardiac function after reperfusion.
Collateral circulation US Medical PG Question 5: A 68-year-old man is brought to the emergency department by ambulance after he was found to be altered at home. Specifically, his wife says that he fell and was unable to get back up while walking to bed. When she approached him, she found that he was unable to move his left leg. His past medical history is significant for hypertension, atrial fibrillation, and diabetes. In addition, he has a 20-pack-year smoking history. On presentation, he is found to still have difficulty moving his left leg though motor function in his left arm is completely intact. The cause of this patient's symptoms most likely occurred in an artery supplying which of the following brain regions?
- A. Motor cortex (ACA) (Correct Answer)
- B. Cerebellum (PICA/SCA)
- C. Occipital cortex (PCA)
- D. Brainstem (Vertebrobasilar)
- E. Motor cortex (MCA)
Collateral circulation Explanation: ***Motor cortex (ACA)***
- The patient's inability to move his **left leg** while his **left arm** remains intact points to an injury in the **right cerebral hemisphere** affecting the leg area of the motor cortex.
- The leg area of the **primary motor cortex** is primarily supplied by the **anterior cerebral artery (ACA)**, making an ACA stroke the most likely cause.
*Motor cortex (MCA)*
- The **middle cerebral artery (MCA)** primarily supplies the motor cortex areas responsible for the **face and arm**, not typically isolated leg weakness.
- If the MCA were affected, you would expect to see involvement of the face and/or arm on the contralateral side in addition to leg weakness.
*Brainstem (Vertebrobasilar)*
- A **brainstem stroke** would likely present with more widespread neurological deficits, including **cranial nerve palsies**, ataxia, or altered consciousness.
- Isolated contralateral leg weakness without arm involvement is not characteristic of a brainstem lesion.
*Cerebellum (PICA/SCA)*
- The **cerebellum** is primarily involved in **coordination and balance**, not direct motor strength.
- A cerebellar stroke would present with symptoms like **ataxia**, dysarthria, or nystagmus, not isolated paralysis.
*Occipital cortex (PCA)*
- The **occipital cortex** is primarily responsible for **vision**.
- A posterior cerebral artery (PCA) stroke would typically cause **visual field defects** (e.g., contralateral homonymous hemianopia) rather than motor weakness.
Collateral circulation US Medical PG Question 6: A 38-year-old woman with moyamoya disease undergoes cerebral angiography showing bilateral progressive stenosis of distal internal carotid arteries with extensive collateral formation. She has had multiple TIAs despite medical management. CT perfusion shows reduced flow in bilateral MCA territories. The neurosurgery team debates between direct bypass (STA-MCA) versus indirect revascularization (EDAS). Evaluate the optimal approach considering Circle of Willis pathophysiology.
- A. Indirect revascularization allows gradual angiogenesis; better for incomplete Circle variants
- B. Combined direct and indirect approaches optimize collateral development regardless of Circle anatomy (Correct Answer)
- C. Circle of Willis anatomy is irrelevant; external to internal carotid bypass is contraindicated in moyamoya
- D. Observation only; intact Circle of Willis through ACoA and PCoA provides adequate collaterals
- E. Direct bypass provides immediate flow; superior for adults with completed Circle
Collateral circulation Explanation: ***Combined direct and indirect approaches optimize collateral development regardless of Circle anatomy***
- In **Moyamoya disease**, the **Circle of Willis** loses its compensatory efficiency due to progressive **internal carotid artery (ICA) stenosis**, necessitating both immediate and long-term revascularization strategies.
- A **combined approach** (direct STA-MCA bypass plus indirect EDAS) provides **immediate hemodynamic augmentation** and facilitates **long-term angiogenesis** to counter chronic ischemia.
*Indirect revascularization allows gradual angiogenesis; better for incomplete Circle variants*
- While **indirect revascularization (EDAS)** promotes gradual **angiogenesis**, it is usually less effective in adults when used alone due to reduced angiogenic potential compared to children.
- It does not address the need for **immediate flow restoration** in a patient experiencing recurrent **TIAs** despite medical management.
*Direct bypass provides immediate flow; superior for adults with completed Circle*
- **Direct bypass** (STA-MCA) does provide **immediate flow**, but relying solely on it ignores the benefit of **indirect synangiosis** in providing wider regional coverage over time.
- The status of a "completed" **Circle of Willis** is often irrelevant in advanced disease because the progressive **distal ICA stenosis** prevents effective cross-flow regardless of anatomical completeness.
*Circle of Willis anatomy is irrelevant; external to internal carotid bypass is contraindicated in moyamoya*
- External to internal carotid bypass is not contraindicated; it is actually a **standard of care** for preventing future **ischemic strokes** in symptomatic Moyamoya patients.
- While the disease originates at the **Circle of Willis**, understanding its anatomy is vital for surgical planning and predicting **collateral compensation** patterns.
*Observation only; intact Circle of Willis through ACoA and PCoA provides adequate collaterals*
- Observation is inappropriate for a patient with **recurrent TIAs** and **reduced CT perfusion**, as it carries a high risk of permanent **ischemic stroke**.
- In **Moyamoya disease**, the typical **distal ICA stenosis** frequently involves the origins of the **ACoA and PCoA**, rendering the **Circle of Willis** inadequate for maintaining cerebral perfusion.
Collateral circulation US Medical PG Question 7: A 70-year-old man undergoes elective clipping of an unruptured basilar tip aneurysm. Preoperative angiography shows bilateral fetal-type posterior cerebral arteries (PCAs arising from internal carotid arteries) with hypoplastic P1 segments. The aneurysm involves both posterior communicating artery origins. Synthesize an approach to surgical planning that optimally preserves cerebral perfusion.
- A. Staged bilateral STA-PCA bypasses followed by aneurysm clipping with PCoA sacrifice
- B. Standard clip placement sacrificing one PCoA; contralateral PCoA provides adequate flow
- C. Bypass surgery from external carotid to PCA prior to aneurysm clipping
- D. Endovascular coiling instead of clipping to preserve both PCoA vessels
- E. Clip reconstruction technique preserving both PCoA origins with intraoperative flow assessment (Correct Answer)
Collateral circulation Explanation: ***Clip reconstruction technique preserving both PCoA origins with intraoperative flow assessment***
- In patients with **fetal-type posterior cerebral arteries (PCAs)**, the **posterior communicating artery (PCoA)** is the primary supply to the PCA territory because the **P1 segment** of the basilar artery is hypoplastic.
- **Clip reconstruction** is the gold standard for maintaining the integrity of these vital vessels, and **intraoperative flow assessment** (like ICG angiography) ensures that perfusion to the occipital and temporal lobes is maintained.
*Staged bilateral STA-PCA bypasses followed by aneurysm clipping with PCoA sacrifice*
- While **bypass surgery** can provide additional flow, performing staged bilateral procedures is excessively invasive and carries a high risk for an **unruptured aneurysm**.
- The goal should be the **anatomic preservation** of the primary vessels rather than replacing them with complex extracranial-to-intracranial bypasses unless necessary.
*Standard clip placement sacrificing one PCoA; contralateral PCoA provides adequate flow*
- Sacrificing a **fetal-type PCoA** is contraindicated because the **hypoplastic P1** cannot provide compensatory flow, leading to a major **ischaemic stroke** in the PCA territory.
- Unlike a typical Circle of Willis, **contralateral flow** cannot cross over to support the deprived PCA territory in this specific anatomical configuration.
*Bypass surgery from external carotid to PCA prior to aneurysm clipping*
- External carotid-to-PCA bypass is a complex rescue operation and is typically reserved for cases where **direct clip reconstruction** is technically impossible.
- In elective surgery for a basilar tip aneurysm, the priority is **microsurgical technique** to save the native **internal carotid-driven** flow.
*Endovascular coiling instead of clipping to preserve both PCoA vessels*
- While **endovascular coiling** is an alternative, the question specifically asks for a **surgical planning approach** based on the decision to use clipping.
- Furthermore, coiling a wide-necked **basilar tip aneurysm** often requires stents or balloons that could still inadvertently compromise the **PCoA origins** if not performed meticulously.
Collateral circulation US Medical PG Question 8: A 42-year-old woman presents with progressive cognitive decline, early-onset dementia, and recurrent subcortical strokes. Genetic testing reveals CADASIL (Cerebral Autosomal Dominant Arteriopathy with Subcortical Infarcts and Leukoencephalopathy). MR angiography shows no large vessel abnormalities but extensive white matter changes. Her sister, also affected, is considering pregnancy. Evaluate the counseling regarding Circle of Willis anatomy and implications for disease manifestations.
- A. Pregnancy increases stroke risk regardless of Circle anatomy; recommend avoiding pregnancy
- B. Complete Circle of Willis anatomy predicts milder phenotype due to better collateral flow
- C. CADASIL primarily affects perforating arteries; Circle of Willis anatomy has minimal impact on phenotype (Correct Answer)
- D. Incomplete Circle increases stroke risk; recommend prophylactic aspirin during pregnancy
- E. Circle of Willis variants determine specific infarct patterns; genetic testing can predict locations
Collateral circulation Explanation: ***CADASIL primarily affects perforating arteries; Circle of Willis anatomy has minimal impact on phenotype***
- **CADASIL** is caused by **NOTCH3 mutations** that lead to the accumulation of granular osmiophilic material in **small penetrating arteries** and arterioles.
- Since the pathology involves **microvasculature** rather than large-caliber vessels, the macrostructure of the **Circle of Willis** does not provide effective collateral flow to the affected deep white matter areas.
*Pregnancy increases stroke risk regardless of Circle anatomy; recommend avoiding pregnancy*
- While CADASIL patients may have an increased risk of stroke or **migraines with aura** during the postpartum period, CADASIL is not an absolute contraindication to pregnancy.
- Genetic counseling should focus on the **50% inheritance risk** (autosomal dominant) rather than suggesting the Circle of Willis anatomy dictates pregnancy outcomes.
*Complete Circle of Willis anatomy predicts milder phenotype due to better collateral flow*
- The **Circle of Willis** provides collateralization for **large vessel** occlusions; however, CADASIL is a **small vessel disease** where collateral circulation is limited at the capillary level.
- Clinical phenotypic severity is more closely linked to **age**, **hypertension**, and the specific **NOTCH3 mutation** site rather than large vessel anatomy.
*Incomplete Circle increases stroke risk; recommend prophylactic aspirin during pregnancy*
- **Ischemic events** in CADASIL are due to intrinsic degenerative changes in the arterial wall (vasculopathy), not necessarily large-vessel flow dynamics influenced by the **Circle of Willis**.
- There is limited evidence to support prophylactic **aspirin** specifically for pregnancy management in CADASIL based solely on vascular variants.
*Circle of Willis variants determine specific infarct patterns; genetic testing can predict locations*
- **Infarct patterns** in CADASIL are typically localized to the **periventricular white matter**, **basal ganglia**, and **thalamus**, irrespective of large vessel variants.
- **Genetic testing** identifies the mutation but cannot accurately predict the exact anatomical location or timing of future **lacunar infarcts**.
Collateral circulation US Medical PG Question 9: A 55-year-old man with diabetes and hypertension undergoes carotid endarterectomy for critical left internal carotid stenosis. Postoperatively, he develops right homonymous hemianopia without motor deficits. Intraoperative transcranial Doppler had shown reduced flow in the ipsilateral middle cerebral artery but preserved anterior cerebral artery flow. Analyze the most likely mechanism for this specific deficit pattern.
- A. Embolic shower to left middle cerebral artery branches during manipulation
- B. Inadequate collateral flow through anterior communicating artery during clamping
- C. Watershed infarct between ACA and MCA territories
- D. Hypoperfusion of posterior cerebral artery via inadequate posterior communicating artery (Correct Answer)
- E. Occlusion of lenticulostriate arteries from perioperative hypotension
Collateral circulation Explanation: ***Hypoperfusion of posterior cerebral artery via inadequate posterior communicating artery***
- **Right homonymous hemianopia** without motor deficits localization strongly points to the **left occipital lobe**, which is supplied by the **posterior cerebral artery (PCA)**.
- During **carotid endarterectomy**, if the PCA is dependent on the internal carotid artery (ICA) via a dominant **posterior communicating artery (PCoA)**, clamping can lead to isolated visual field loss if collateral flow is insufficient.
*Embolic shower to left middle cerebral artery branches during manipulation*
- **Emboli** to the **middle cerebral artery (MCA)** typically result in cortical deficits like hemiparesis or aphasia, which are absent in this patient.
- Transcranial Doppler noted reduced flow but not the micro-embolic signals characteristic of an **embolic shower**.
*Inadequate collateral flow through anterior communicating artery during clamping*
- The **anterior communicating artery (ACoA)** primarily supplies the **anterior cerebral artery (ACA)** territory, and flow there was documented as preserved.
- Deficits here would typically manifest as **lower limb weakness** rather than a pure homonymous hemianopia.
*Watershed infarct between ACA and MCA territories*
- **Watershed infarcts** between these territories (the 'man-in-a-barrel' zone) cause **proximal limb weakness**, not isolated visual field defects.
- The pattern would not explain a complete **homonymous hemianopia**, which requires damage to the optic radiation or **visual cortex**.
*Occlusion of lenticulostriate arteries from perioperative hypotension*
- **Lenticulostriate artery** occlusion results in a lacunar stroke of the **internal capsule**, causing dense motor deficits.
- This patient specifically had **no motor deficits**, which excludes involvement of the deep perforating branches of the MCA.
Collateral circulation US Medical PG Question 10: A 28-year-old woman presents with bilateral lower extremity weakness worse than upper extremity weakness, abulia, and urinary incontinence. MRI reveals bilateral anterior cerebral artery territory infarcts. Angiography shows an azygos anterior cerebral artery variant. Analyze the anatomical basis for this patient's bilateral deficits from a presumed single vascular occlusion.
- A. Both A1 segments are hypoplastic with dominant left A2 segment
- B. Single midline A2 segment supplies both hemispheres without ACoA (Correct Answer)
- C. Bilateral A1 occlusions with isolated posterior circulation
- D. ACoA aneurysm with mass effect on both A2 segments
- E. Duplicate anterior communicating arteries with bilateral flow restriction
Collateral circulation Explanation: ***Single midline A2 segment supplies both hemispheres without ACoA***
- An **azygos anterior cerebral artery (ACA)** is a rare anatomical variant where both A1 segments merge to form a **single midline A2 trunk**, which then supplies the medial aspects of both cerebral hemispheres.
- A single embolic or thrombotic occlusion in this **common trunk** results in bilateral infarction, explaining the patient's **bilateral lower extremity weakness**, **abulia**, and **urinary incontinence**.
*Both A1 segments are hypoplastic with dominant left A2 segment*
- Hypoplasia of both A1 segments would likely lead to recruitment of blood from the **posterior circulation** via the **posterior communicating arteries** rather than a single infarct resulting in bilateral deficits.
- A dominant A2 segment usually implies an **unpaired artery of Wilder**, but the classic "azygos" description specifically refers to a **midline shared vessel**.
*Bilateral A1 occlusions with isolated posterior circulation*
- Simultaneously occurring **bilateral A1 occlusions** are extremely rare and do not represent a single vascular variant as described by the angiography.
- This scenario would likely present with more profound global ischemia rather than the specific **azygos ACA** distribution mentioned.
*ACoA aneurysm with mass effect on both A2 segments*
- While an **anterior communicating artery (ACoA)** aneurysm can cause focal deficits, it typically presents with **subarachnoid hemorrhage** or cranial nerve palsies rather than sudden-onset ischemic infarcts in a specific vascular territory.
- Mass effect is a chronic or subacute presentation and does not explain the **angiographic appearance** of an azygos segment.
*Duplicate anterior communicating arteries with bilateral flow restriction*
- **Duplicate ACoAs** are relatively common variants but would provide **redundant collateral flow**, potentially protecting the brain from bilateral infarction rather than causing it.
- Bilateral flow restriction across two separate arteries simultaneously is not a likely outcome of a **single vascular occlusion** event.
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