What is the most probable cause of oligoanuria in a patient, potentially with a history of diabetes, considering options such as unilateral obstructing nephrolithiasis, acute prostatitis, neurogenic bladder, or dehydration?

Most Probable Cause of Oligoanuria

Dehydration is the most probable cause of oligoanuria among the options presented, as it represents a prerenal cause that accounts for the vast majority of acute kidney injury cases, while unilateral obstructing nephrolithiasis alone cannot cause oligoanuria unless both kidneys are affected or only one functional kidney exists. 1

Critical Pathophysiologic Distinction

Unilateral obstruction does NOT cause oligoanuria because the contralateral kidney maintains adequate urine output. 1 Postrenal causes of acute kidney injury require bilateral ureteral obstruction, bladder outlet obstruction, or obstruction of a solitary functioning kidney to produce oligoanuria. 1 Renal and prerenal etiologies account for >97% of acute kidney injury cases, with obstruction being relatively uncommon. 1

Why Dehydration is Most Likely

Mechanism and Prevalence

• Dehydration causes prerenal azotemia through reduced renal perfusion, leading to decreased glomerular filtration rate and oliguria. 1, 2
• Prerenal causes including hypovolemia and decreased cardiac output are among the most common etiologies of acute kidney injury. 1
• In dehydration, reduced intravascular volume triggers enhanced urea reabsorption in the proximal tubule, creating a disproportionate BUN-to-creatinine ratio elevation (typically >20:1). 2

Clinical Recognition

• Oliguria from dehydration is typically reversible with fluid resuscitation, unlike intrinsic renal causes. 3, 4
• Hypovolemic oliguric patients increase urine output to >0.5 mL/kg/h following a 500-mL normal saline bolus. 4
• The absence of other markers of kidney injury (proteinuria, hematuria, abnormal urinary sediment) helps distinguish dehydration from intrinsic kidney disease. 2

Why Other Options Are Less Likely

Acute Prostatitis

• While acute prostatitis can cause bladder outlet symptoms, it rarely causes complete urinary obstruction severe enough to produce oligoanuria. 1
• Bladder outlet obstruction would need to be severe and bilateral (affecting both ureters through bladder distention) to cause oligoanuria. 1

Neurogenic Bladder from Diabetes

• Diabetic neurogenic bladder develops gradually and is characterized by increased residual urine volume, not oligoanuria. 5, 6
• Early diabetic bladder dysfunction shows impaired sensation and incomplete emptying without residual urine, infection, or azotemia. 6
• The progression to decompensation with significant residual urine is the measure of advancing bladder neuropathy, but this still produces urine—it simply retains it in the bladder. 6
• Chronic kidney disease prevalence in neurogenic bladder patients is elevated (22.4% vs general population), but this reflects chronic deterioration, not acute oligoanuria. 5

Unilateral Obstructing Nephrolithiasis

• This cannot cause oligoanuria unless the patient has only one functioning kidney. 1
• The contralateral kidney compensates and maintains normal urine output. 1
• Postrenal acute kidney injury requires bilateral obstruction or obstruction of a solitary kidney. 1

Clinical Pitfalls to Avoid

• Do not assume unilateral obstruction causes oligoanuria—always verify the function of the contralateral kidney. 1
• Oliguria is common in critically ill patients (18% incidence) and frequently results from renal hypoperfusion rather than intrinsic kidney disease. 4
• Oliguria without creatinine elevation still carries increased mortality risk (8.8% ICU mortality) compared to non-oliguric patients (1.3%). 7
• In diabetic patients, serum creatinine may underestimate renal dysfunction due to reduced muscle mass from neuropathy. 5

Diagnostic Approach

Assess volume status first through clinical examination (skin turgor, mucous membranes, orthostatic vital signs) and laboratory markers (BUN:creatinine ratio >20:1 suggests prerenal cause). 2, 4

Administer a 500-mL normal saline fluid challenge:

• If urine output increases to >0.5 mL/kg/h, dehydration is confirmed. 4
• If oliguria persists despite adequate fluid resuscitation, consider intrinsic renal causes or true obstruction. 4

Check urinary indices in oliguric patients:

• Urine sodium <20 mEq/L and fractional excretion of sodium <1% suggest prerenal azotemia from dehydration. 4
• Urine sodium >40 mEq/L and fractional excretion of sodium >2% suggest intrinsic renal disease. 4

Obtain renal ultrasound only if:

• Oligoanuria persists after adequate rehydration. 1
• Clinical suspicion exists for bilateral obstruction or solitary kidney obstruction. 1
• Patient has known single kidney or history of contralateral nephrectomy. 1