Which laboratory abnormality is most expected in a 28-year-old man with hypovolemic acute kidney injury due to excessive military training?

Laboratory Findings in Hypovolemic Acute Kidney Injury

In a 28-year-old man with hypovolemic AKI from excessive military training, the most expected laboratory abnormality is an elevated BUN/creatinine ratio >20 (Option C). 1

Pathophysiology of Prerenal Azotemia

In hypovolemic states, the kidneys respond by maximizing sodium and water reabsorption to preserve intravascular volume. 2 This physiologic response creates a characteristic laboratory pattern:

• BUN/creatinine ratio >20:1 is the hallmark finding, as enhanced proximal tubular reabsorption of water increases passive urea reabsorption disproportionately to creatinine 1
• The kidneys increase reabsorption of both sodium and water to maintain intravascular volume, resulting in concentrated urine with low sodium content 2
• This represents appropriate kidney function responding to decreased renal perfusion, not intrinsic kidney damage 2

Why Other Options Are Incorrect

Option A (Low urine osmolarity <250 mOsm/L) is incorrect:

• Hypovolemic patients concentrate their urine maximally, producing high urine osmolarity (typically >500 mOsm/L), not low 2
• Low urine osmolarity suggests inability to concentrate urine, which occurs in intrinsic renal disease (ATN), not prerenal states 2

Option B (FENa >2%) is incorrect:

• Prerenal azotemia characteristically shows FENa <1%, reflecting avid sodium retention 2
• FENa >2% indicates tubular dysfunction and suggests acute tubular necrosis, not hypovolemia 2
• In this clinical scenario of excessive training with dehydration, the kidneys should be avidly retaining sodium 2

Option D (Dysmorphic RBCs and RBC casts) is incorrect:

• These findings indicate glomerular disease or glomerulonephritis 1
• Hypovolemic AKI produces a "bland" or normal urine sediment without cellular elements 2
• The presence of >50 RBCs/hpf or proteinuria >500 mg/day would suggest glomerular involvement, which is not consistent with this clinical presentation 1

Clinical Context Supporting BUN/Creatinine Ratio Elevation

This patient's presentation is classic for prerenal azotemia:

• Recent excessive military training suggests significant fluid losses through sweating 3
• Postural hypotension confirms volume depletion 3
• Muscle cramping and fatigue are consistent with dehydration and electrolyte disturbances 3

Important Caveats

The BUN/creatinine ratio has limitations:

• While traditionally used to distinguish prerenal from intrinsic AKI, approximately half of AKI patients have elevated BUN/creatinine ratios regardless of etiology 4
• High protein intake, gastrointestinal bleeding, or hypercatabolic states can elevate BUN disproportionately even without true prerenal physiology 5
• However, in this specific clinical context of a young, previously healthy individual with clear volume depletion from excessive training, the elevated ratio reliably indicates prerenal azotemia 6

Additional expected findings in this patient:

• Urine sodium <10 mEq/L 2
• Urine osmolality >500 mOsm/L 2
• FENa <1% 2
• Normal or bland urine sediment 2

The combination of clinical presentation (excessive training, postural hypotension) with laboratory confirmation (elevated BUN/creatinine ratio) establishes the diagnosis of hypovolemic prerenal AKI. 1, 2