A 72-year-old man with longstanding history of diabetes mellitus and hypertension presents to the emergency department with sudden-onset numbness. On your neurological exam, you note that he has loss of sensation on the left side of his face, arm, and leg. His motor strength exam is normal, as are his cranial nerves. Which of the following is the most likely explanation for his presentation?

Anterior cerebral artery stroke

A 35-year-old man who suffered a motor vehicle accident 3 months ago presents to the office for a neurological evaluation. He has no significant past medical history and takes no current medications. He has a family history of coronary artery disease in his father and Alzheimer’s disease in his mother. On physical examination, his blood pressure is 110/60 mm Hg, the pulse is 85/min, the temperature is 37.0°C (98.6°F), and the respiratory rate is 20/min. Neurological examination is suggestive of a lesion in the anterior spinal artery that affects the anterior two-thirds of the spinal cord, which is later confirmed with angiography. Which of the following exam findings would have suggested this diagnosis?

Loss of vibratory sense below the level of the lesion

Loss of pain and temperature sensation above the level of the lesion

Negative plantar extensor response in his lower limbs

Flaccid paralysis on the right side

A 36-year-old woman, gravida 1, para 1, has back pain and numbness in her lower extremities after an emergency cesarean delivery of a healthy 3856-g (8-lb, 8-oz) newborn male. She had a placental abruption and lost approximately 2000 ml of blood. During the procedure, she received two units of packed red blood cells and intravenous fluids. She has no history of serious illness and takes no medications. She is sexually active with one male partner, and they use condoms inconsistently. She is alert and oriented to person, place, and time. Her temperature is 37.2°C (98.9°F), pulse is 90/min, respirations are 15/min, and blood pressure is 94/58 mm Hg. Examination shows decreased sensation to temperature and pinprick below her waist and 0/5 muscle strength in her lower extremities. She feels the vibrations of a tuning fork placed on both of her great toes. Deep tendon reflexes are absent in the lower extremities and 2+ in the upper extremities. Which of the following is the most likely diagnosis?

Anterior spinal artery syndrome

Posterior spinal artery syndrome

A 65-year-old man presents with facial weakness. He says he noticed that his face appeared twisted when he looked in the bathroom mirror this morning. He is otherwise well and does not have any other complaints. He denies any facial pain or paresthesia. No significant past medical history. The patient is afebrile and vital signs are within normal limits. Neurological examination reveals difficulty shutting the right eye tight and inability to bring up the right corner of his mouth when asked to smile. Remainder of the exam, including the left side of the face, is unremarkable. Which of the following is the most likely diagnosis in this patient?

Left middle cerebral artery stroke

A 38-year-old man comes to the physician for a follow-up examination. He has quadriparesis as a result of a burst fracture of the cervical spine that occurred after a fall from his roof 1 month ago. He has urinary and bowel incontinence. He appears malnourished. His temperature is 37.1°C (98.8°F), pulse is 88/min, and blood pressure is 104/60 mm Hg. Examination shows spasticity in all extremities. Muscle strength is decreased in proximal and distal muscle groups bilaterally. Deep tendon reflexes are 4+ bilaterally. Plantar reflex shows extensor response bilaterally. Sensation to pinprick and temperature is absent below the neck. Sensation to vibration, position, and light touch is normal bilaterally. Rectal tone is decreased. There is a 1-cm area of erythema over the sacrum. Which of the following is the most likely cause of this patient's symptoms?

Damage to the anterior spinal artery

Hemi-transection of the spinal cord

Cavitation within the spinal cord

Injury to gray matter of the spinal cord

Occlusion of the posterior spinal artery

Anterior cerebral artery territory for USMLE Step 2 CK: High-Yield Anatomy Lessons

ACA: Anatomy & Course - The Brain's Mohawk

Origin: Arises from the internal carotid artery (ICA) bifurcation.
Path: Travels anteriorly to enter the longitudinal fissure, arching superiorly and posteriorly over the corpus callosum.
- This course along the brain's midline gives it the "mohawk" distribution.
Segments:
- A1 (Pre-communicating): From ICA to the anterior communicating artery (AComA). Gives rise to medial lenticulostriate arteries.
- A2 (Post-communicating): Distal to AComA; arches over the corpus callosum.

⭐ The A1 segment is typically hypoplastic in ~10% of individuals, making the contralateral A1 the sole supply for both ACAs.

Brain vascular supply: ACA, MCA, PCA territories

ACA: Cortical Territories - Mind & Motor Strip

Location: Supplies the medial surface of the frontal and parietal lobes, including the superior frontal gyrus and paracentral lobule.
Key Structures & Functions:
- Motor Cortex: Controls the contralateral leg and foot.
- Sensory Cortex: Sensation for the contralateral leg and foot.
- Prefrontal Cortex: Governs executive function, judgment, and social behavior.
- Paracentral Lobule: Critical for voluntary control of urination.
Clinical Picture (ACA Stroke):
- Contralateral paralysis & sensory loss, predominantly in the lower limb.
- Urinary incontinence.
- Personality changes: abulia (apathy), disinhibition, impaired judgment.

⭐ Exam Classic: ACA strokes characteristically spare the upper limb and face, which are supplied by the MCA. This lower limb vs. upper limb/face deficit is a key distinguishing feature.

📌 Mnemonic: "A.C.A."

Apathy / Abulia
Contralateral leg weakness
Abnormal micturition (incontinence)

Brain vascular territories: ACA, MCA, PCA

ACA: Stroke Syndrome - Contralateral Leg Day

Brain vascular territories, sagittal and coronal views

Vascular Territory: Supplies the medial (mesial) surface of the frontal and parietal lobes, including the leg area of the motor and sensory homunculi, and the anterior four-fifths of the corpus callosum.
Core Manifestations: Think "Contralateral Leg Day."
- Motor: Contralateral paralysis or paresis, predominantly affecting the lower limb. Gait apraxia is common.
- Sensory: Contralateral sensory loss, also confined to the lower limb.
Frontal Lobe Signs: Often prominent.
- Abulia: A profound apathy and lack of motivation.
- Urinary Incontinence: Loss of cortical inhibition of the bladder.
- Re-emergence of primitive (frontal release) signs like the grasp or sucking reflex.

⭐ Exam Pearl: Unlike MCA strokes, ACA strokes spare the face and arms. The presence of frontal lobe signs (especially abulia and incontinence) with pure leg weakness is highly suggestive of an ACA territory infarct.

📌 Mnemonic: A in ACA for Abulia, Apathy, and Akinesia of the contralateral leg.

ACA: Branches & Aneurysms - Danger at the Crossroads

Key Branches:
- Cortical: Supply medial frontal & parietal lobes (motor/sensory cortex for lower limb).
- Penetrating (Medial Lenticulostriate): Supply anterior limb of internal capsule, caudate nucleus.
Aneurysm Hotspot:
- Anterior Communicating Artery (AComm): Most common site of saccular (berry) aneurysms.
- Rupture leads to subarachnoid hemorrhage (SAH) and potential frontal lobe deficits.

⭐ AComm aneurysm rupture can compress the optic chiasm, causing bitemporal hemianopia.

Circle of Willis diagram with labeled arteries

High‑Yield Points - ⚡ Biggest Takeaways

ACA strokes classically affect the medial portions of the frontal and parietal lobes.

Presents with contralateral paralysis and sensory loss of the lower limb (leg and foot).

Key frontal lobe signs include personality changes (e.g., abulia) and the re-emergence of primitive reflexes.

Urinary incontinence is a highly specific finding due to damage to the paracentral lobule.

Damage to the corpus callosum can lead to interhemispheric disconnection syndromes like apraxia.

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