A 25-year-old army recruit is brought into the emergency room after his first week of summer boot camp complaining of dizziness and decreased urination of 2 days duration. Physical exam shows a lethargic young male with sunken eyes and chapped lips. Vital signs are significant for a pulse of 120 bpm and a blood pressure of 100/60. Blood tests show a BUN of 45 mg/dL and a creatinine of 2 mg/dL. Urine analysis reveal a fractional excreted sodium of <1%. An abnormality in which of the following is the most likely cause for this patient’s condition?
1.Functional status of the urea cycle
2.Glomerular filtration rate (GFR)
3.Myocardial contractility
4.Protein content in the diet
5.Proximal tubule reabsorption
Correct answer
Glomerular filtration rate (GFR)
Explanation
This patient’s clinical scenario is characteristic of acute renal failure secondary to hypovolemia. Acute renal failure (ARF) can manifest clinically as oliguria (100-400 ml in 24 hours) or anuria (<100 ml in 24 hours). ARF can be classified as pre-renal, intrinsic, or post-renal. Pre-renal ARF can be defined with labs showing BUN/Cr >20 and fractional excreted sodium <1%. Hypovolemia is a common cause of pre-renal ARF and can be identified clinically by signs of dehydration: dizziness, sunken eyes, dry mucous membranes, tachycardia, and hypotension. In this dehydrated patient, ARF is a result of decreased cardiac output from hypovolemia leading to a decrease in GFR (choice B).
Problems with the urea cycle (choice A) could explain the patient’s elevated BUN, but it would not explain his elevated creatinine and renal failure.
There is no evidence that this patient’s myocardial contractility (choice C) is compromised. His tachycardia is likely due to decreased preload, secondary to hypovolemia. In fact, good myocardial contractility is allowing him to maintain a systolic blood pressure of 100 despite significant hypovolemia.
Dietary protein (choice D) can increase serum BUN, but it is not a potential etiology of ARF.
Dysfunction of proximal tubule reabsorption (choice E) is unlikely in this patient as demonstrated by his elevated BUN/Cr ratio and low fractional excreted sodium. Urea is reabsorbed by the proximal tubule while creatinine is not; thus, if the proximal tubule were damaged, the BUN/Cr ratio would be smaller. Additionally, the proximal tubule also reabsorbs sodium. Damaged proximal tubules would result in a larger fractional excreted sodium.