Differential Diagnosis for Sudden Onset Left Leg Weakness

Single Most Likely Diagnosis

• Right anterior cerebral artery (ACA) infarction: The patient's symptoms of left leg weakness, diminished sensation over the left leg, and flat affect suggest a lesion in the right hemisphere of the brain, specifically in the territory supplied by the ACA, which is responsible for lower limb function.

Other Likely Diagnoses

• Right middle cerebral artery (MCA) infarction with ACA involvement: Although the MCA primarily supplies the lateral aspect of the hemisphere, including areas responsible for arm and face function, its territory can variably include parts of the leg area, especially if there's a significant ACA-MCA watershed involvement.
• Left lenticulostriate arteries infarction: These arteries supply the basal ganglia and internal capsule, areas that can affect motor function. However, the pattern of weakness and sensory loss might not as typically match the described symptoms as an ACA infarction would.

Do Not Miss Diagnoses

• Bilateral anterior cerebral artery infarctions: Although less likely given the unilateral nature of the symptoms, bilateral ACA infarctions could lead to more profound leg weakness and potentially altered mental status, making it a critical diagnosis not to miss due to its implications for prognosis and management.
• Vertebral or basilar artery occlusion with subsequent ACA infarction: Though the primary symptoms suggest ACA territory involvement, the possibility of a more proximal occlusion affecting multiple territories, including the ACA through collateral circulation, should be considered, especially if there are signs of brainstem or posterior cerebral artery territory involvement.

Rare Diagnoses

• Moyamoya disease with ACA territory infarction: A rare condition causing stenosis or occlusion of the internal carotid artery and its branches, leading to ischemic symptoms. It's less likely in this age group without other suggestive history but should be considered in younger patients or those with a suggestive clinical context.
• Vasculitis affecting the ACA: Inflammatory diseases affecting blood vessels can lead to ischemic symptoms by causing vascular occlusion. This would be an unusual presentation without other systemic symptoms or laboratory findings suggestive of vasculitis.