A 75-year-old man is brought to the emergency department by his family due to worsening co… — USMLE Step 2 CK

A 75-year-old man is brought to the emergency department by his family due to worsening confusion and severe lower abdominal pain over the past 24 hours. His family reports he has not urinated since yesterday morning. His medical history is significant for benign prostatic hyperplasia (BPH) and bipolar disorder, for which he takes tamsulosin and lithium. On presentation, his blood pressure is $155/95\text{ mm Hg}$ , and his heart rate is $98/\text{min}$ . Physical examination reveals a firm, tender suprapubic mass extending to the umbilicus.

Laboratory studies reveal:
Blood urea nitrogen: $88\text{ mg/dL}$
Serum creatinine: $4.8\text{ mg/dL}$ (baseline $1.1\text{ mg/dL}$ )
Serum potassium: $6.3\text{ mEq/L}$
Urinalysis: negative for leukocyte esterase and nitrites

A urethral Foley catheter is placed, immediately yielding $1,800\text{ mL}$ of clear urine. He is admitted for monitoring. Over the next 4 hours, his urine output consistently remains between $1,000\text{ mL}$ and $1,200\text{ mL}$ per hour. His repeat blood pressure drops to $84/52\text{ mm Hg}$ , and his heart rate increases to $122/\text{min}$ . The patient appears pale, and his mucous membranes are severely dry.

Which of the following is the most appropriate next step in the management of this patient?

Initiate intravenous fluid replacement with $0.45\%$ sodium chloride targeting replacement of a portion of the hourly urine outputAnswer
B
Administer broad-spectrum intravenous antibiotics, such as piperacillin-tazobactam, for suspected septic shock
C
Transport the patient for an emergent computed tomography (CT) scan of the abdomen and pelvis to rule out bladder rupture
D
Administer intravenous sodium bicarbonate to empirically treat severe uremic metabolic acidosis
E
Initiate a selective serotonin reuptake inhibitor (SSRI) to manage his acute agitation and delirium

Answer

The most appropriate next step is to initiate intravenous fluid replacement with

0.45\%

sodium chloride targeting replacement of a portion of the hourly urine output.

The correct action addresses the immediate life-threatening physiological derangement. Following the relief of prolonged urinary tract obstruction, the kidneys can produce massive amounts of urine due to osmotic diuresis (from retained urea) and a temporary insensitivity to antidiuretic hormone (nephrogenic diabetes insipidus). This is known as post-obstructive diuresis. If the hourly urine output exceeds

>200\text{ mL/hr}

, patients can rapidly lose profound amounts of intravascular volume. In this scenario, the patient lost over

4\text{ Liters}

of fluid in just 4 hours, culminating in severe hypovolemic shock. The strict standard of care is aggressive intravenous fluid replacement, most commonly using

0.45\%

sodium chloride to account for the large free water deficit, replacing approximately

50\%

75\%

of the hourly urinary output until the diuresis spontaneously resolves.

Step-by-Step Solution

Analyze the patient's initial presentation.

The patient presented with acute urinary retention secondary to BPH, causing severe post-renal acute kidney injury (creatinine

4.8\text{ mg/dL}

Identifying the initial pathology establishes the baseline necessary to interpret the subsequent complication.

Evaluate the clinical course immediately following catheterization.

The patient produced massive amounts of urine (

1,000

1,200\text{ mL/hr}

), leading to tachycardia (

122/\text{min}

) and hypotension (

84/52\text{ mm Hg}

This rapid, high-volume fluid loss confirms the diagnosis of post-obstructive diuresis.

Determine the physiological consequence of the diuresis.

The uncompensated fluid loss has plunged the patient into severe hypovolemic shock.

Understanding the mechanism of shock dictates the necessary immediate therapeutic intervention.

Select the most appropriate life-saving intervention.

Aggressive intravenous fluid replacement, typically with hypotonic fluids like

0.45\%

sodium chloride.

Fluids must be administered to match the excessive urinary losses (often replacing

50\%

75\%

of the hourly output) to restore hemodynamic stability until the kidneys recover their concentrating ability.

Key Concept

Post-obstructive diuresis is a potentially life-threatening condition following the relief of bilateral urinary tract obstruction, requiring aggressive intravenous fluid replacement (often with

0.45\%

normal saline) to prevent severe hypovolemic shock and electrolyte derangements.

Estimated Time:1m 30s

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