In a 26-week gestation fetus, ultrasound shows a dilated left ventricle filled with echogenic material while the right ventricle is normal. What is the most likely diagnosis?

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The key idea here is that the severity of intraventricular hemorrhage is determined by whether blood is confined to the germinal matrix, spills into the ventricles, and whether the ventricles become dilated or there is parenchymal injury. In a very premature fetus, the germinal matrix vessels are especially fragile, so bleeding can occur and extend into the ventricular system.

Seeing a dilated left ventricle filled with echogenic material means blood has entered the ventricular cavity and caused dilation, with a clot present inside the ventricle. That pattern fits an intraventricular hemorrhage with ventricular clot and hydrocephalus, which is grading corresponding to Grade III. Grade II would be IVH without ventricular dilation, Grade I would be confined to the germinal matrix, and Grade IV would involve parenchymal (brain tissue) hemorrhage beyond the ventricle. Subependymal hemorrhage would show bleeding near the ventricular lining rather than a clot filling the ventricle itself.

So, the unilateral ventricular dilation with intraventricular echogenic clot in a 26-week fetus best indicates an intraventricular hemorrhage with ventricular involvement and clot—Grade III IVH.