A 67 year old male presents to the ER with a creatinine of 450 m/L. His JVP is low, his mucous membranes are dry and he has postural tachycardia and hypotension. He is felt to be volume depleted and IV normal saline is started. An abdomal ultrasound shows no evidence of hydronephrosis or hydroureter. 3 days later, the patient is clinically euvolemic but the serum creatinine is still 400 m/L.
Has obstruction been truly ruled out? Provide two explanations describing how the patient could be obstructed despite an unremarkable ultrasound on admission.