A 42-year-old man is evaluated in the emergency department for a 3-week history of progres… — USMLE Step 2 CK

A $42$ -year-old man is evaluated in the emergency department for a $3$ -week history of progressively worsening right upper quadrant abdominal pain, high fevers, and a dry cough. He immigrated from a rural region of Central America $12$ years ago and returned there to visit relatives for a month, arriving back in the United States two months ago. His temperature is $39.2^\circ\text{C}$ ( $102.6^\circ\text{F}$ ), blood pressure is $110/70$ mm Hg, pulse is $112$ /min, and respirations are $24$ /min. Physical examination reveals scleral icterus and significant point tenderness over the right lower rib cage. Dullness to percussion is noted at the right lung base.

Arterial blood gas analysis on room air shows:

Parameter	Value
pH	$7.50$
pCO $_2$	$28$ mm Hg
pO $_2$	$85$ mm Hg

Additional laboratory studies show:

Test	Result
Leukocyte count	$17,500/\text{mm}^3$
Total bilirubin	$2.8$ mg/dL
Alkaline phosphatase	$450$ U/L
Aspartate aminotransferase (AST)	$85$ U/L
Alanine aminotransferase (ALT)	$92$ U/L

An abdominal ultrasound demonstrates a solitary, $8$ -cm, hypoechoic lesion in the right hepatic lobe. To rule out a pyogenic source, an ultrasound-guided aspiration is performed, yielding a thick, odorless, dark-brown fluid. Gram stain shows abundant leukocytes but no organisms.

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

Intravenous metronidazole followed by oral paromomycinAnswer
B
Intravenous piperacillin-tazobactam and placement of a percutaneous drainage catheter
C
Exploratory laparotomy and surgical unroofing of the hepatic lesion
D
Administration of acetazolamide to correct the acid-base abnormality
E
Emergent transport for multiphasic abdominal computed tomography

Answer

The correct answer is to initiate intravenous metronidazole followed by oral paromomycin.

The correct answer identifies the need for biphasic medical therapy. The patient has a classic presentation of an amebic liver abscess caused by Entamoeba histolytica. The epidemiologic history of recent travel to an endemic area, combined with high fever, right upper quadrant pain, and a solitary right lobe lesion, strongly suggests this diagnosis. The sympathetic right-sided pleural effusion (causing dullness to percussion and a pain-driven respiratory alkalosis) is a frequent adjacent complication. The pathognomonic finding is the aspirate of "anchovy paste" fluid (thick, dark-brown, and odorless) that remains sterile on standard bacterial culture. The definitive treatment for an amebic liver abscess is a tissue amebicide, such as intravenous metronidazole, which rapidly clears the hepatic infection. Crucially, because E. histolytica cysts can remain dormant in the intestinal lumen and cause future relapses, a luminal agent (such as paromomycin or iodoquinol) must subsequently be administered to achieve complete eradication.

Step-by-Step Solution

Analyze the epidemiologic and clinical presentation.

A young man with recent travel to an endemic area presenting with fever, right upper quadrant pain, hepatomegaly, and a reactive pleural effusion.

Establishing the pre-test probability for tropical infectious etiologies of a liver lesion.

Interpret the diagnostic fluid characteristics.

The thick, odorless, dark-brown "anchovy paste" fluid that is sterile on standard bacterial cultures confirms the diagnosis of an amebic liver abscess.

Differentiating an amebic abscess (Entamoeba histolytica) from a pyogenic abscess or Echinococcal cyst is critical for selecting therapy.

Determine the definitive biphasic pharmacotherapy.

Therapy requires a tissue-active agent (metronidazole) to resolve the hepatic abscess, followed by a luminal agent (paromomycin or iodoquinol).

E. histolytica cysts remain dormant in the intestinal lumen and will cause future relapses if not specifically targeted.

Key Concept

Amebic liver abscesses present with a classic "anchovy paste" aspirate and require biphasic medical therapy: a tissue amebicide (metronidazole) to clear the invasive hepatic infection, followed by a luminal agent (paromomycin) to eradicate intestinal colonization and prevent relapse.

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