Differential Diagnosis for Renal Function Abnormalities

The patient's presentation with elevated blood urea nitrogen (BUN) and serum creatinine, along with a decreased estimated glomerular filtration rate (eGFR), suggests an acute kidney injury (AKI). The following differential diagnoses are considered:

• Single Most Likely Diagnosis

  • D. Acute Prerenal Azotemia: This is the most likely explanation given the patient's initial presentation with hypotension (blood pressure 86/53 mm Hg), tachycardia (pulse rate 134 per minute), and signs of dehydration (dry mucous membranes). The improvement in blood pressure and reduction in tachycardia and tachypnea after fluid resuscitation with lactated Ringer's solution support this diagnosis. The fractional excretion of sodium (FeNa) of 2.5% also points towards a prerenal cause, as it is less than 3%, indicating that the kidneys are appropriately conserving sodium in response to decreased renal perfusion.

• Other Likely Diagnoses

  • A. Acute Tubular Necrosis (ATN): Although less likely than prerenal azotemia given the FeNa and the clinical context, ATN could be considered, especially if there were concerns about ischemic injury to the kidneys during the period of hypotension. However, the rapid improvement in lactate levels and the initial response to fluid resuscitation make this less likely.
  • B. Acute Interstitial Nephritis: This could be a consideration if there were exposures to medications known to cause interstitial nephritis, such as NSAIDs or certain antibiotics. However, the patient reports no recent NSAID use, and there's no mention of other offending medications.

• Do Not Miss Diagnoses

  • Sepsis-related AKI: Although the patient does not have a high fever, sepsis can present with hypotension, tachycardia, and organ dysfunction, including AKI. It's crucial to consider and rule out sepsis, especially in a patient with a severe infection or other risk factors.
  • Obstructive Uropathy: This would be unusual given the lack of specific symptoms or findings suggestive of urinary obstruction, but it's a condition that could lead to renal failure and should be considered if other explanations are not forthcoming.

• Rare Diagnoses

  • C. Acute Glomerulonephritis: This would be an unusual cause of AKI in this context, given the lack of specific findings suggestive of an immune-mediated glomerular disease, such as hematuria, proteinuria, or specific serological markers.
  • Other rare causes of AKI, such as vasculitis, thrombotic microangiopathy, or atheroembolic disease, could be considered if the patient's presentation and initial workup do not lead to a clear diagnosis, but these would be less likely given the information provided.