Emergency medical services bring a primigravid patient at 36 weeks gestation to the obstet… — USMLE Step 2 CK

Emergency medical services bring a primigravid patient at $36$ weeks gestation to the obstetric triage unit due to altered mental status. The $31$ -year-old patient has a two-day history of persistent nausea, vomiting, and right-sided abdominal pain. According to her partner, she became progressively lethargic this morning. Her prenatal course was previously uncomplicated. Vital signs include a blood pressure of $132/84\text{ mmHg}$ , heart rate of $105\text{/min}$ , and temperature of $37.1^\circ\text{C}$ ( $98.8^\circ\text{F}$ ). Physical examination is notable for scleral icterus, dry mucous membranes, and tenderness to palpation in the right upper quadrant without guarding. Continuous fetal monitoring reveals a baseline heart rate of $140\text{/min}$ with moderate variability and no decelerations. Initial laboratory studies reveal:

Test	Value
Hemoglobin	$11.2\text{ g/dL}$
Platelets	$85,000/\text{mm}^3$
Leukocytes	$14,500/\text{mm}^3$
Aspartate aminotransferase (AST)	$650\text{ U/L}$
Alanine aminotransferase (ALT)	$720\text{ U/L}$
Total bilirubin	$4.8\text{ mg/dL}$
Creatinine	$1.8\text{ mg/dL}$
Glucose	$42\text{ mg/dL}$

Which of the following is the primary pathophysiologic mechanism responsible for this patient's most likely diagnosis?

Microvesicular fatty infiltration of hepatocytesAnswer
B
Systemic maternal endothelial dysfunction and vasospasm
C
Defective biliary excretion of bile acids
D
Hepatocellular necrosis mediated by cytotoxic T cells
E
Physiologic hemodilution and altered hepatic blood flow

Answer

The correct answer is microvesicular fatty infiltration of hepatocytes, which is the underlying pathophysiologic mechanism of acute fatty liver of pregnancy (AFLP).

Acute fatty liver of pregnancy (AFLP) is an obstetric emergency characterized pathophysiologically by microvesicular fatty infiltration of hepatocytes. This accumulation of fat leads to rapid, profound hepatocellular dysfunction and synthetic liver failure. It typically presents in the third trimester with nausea, vomiting, abdominal pain, and jaundice. The severe hypoglycemia and altered mental status (encephalopathy) seen in this patient are hallmarks of fulminant liver synthetic failure, which helps clinically differentiate AFLP from other hypertensive and hepatic disorders of pregnancy. Immediate maternal stabilization and prompt delivery are required.

Step-by-Step Solution

Analyze the patient's clinical presentation.

The patient is in the third trimester presenting with nausea, vomiting, right upper quadrant pain, jaundice, and altered mental status (lethargy/confusion).

Identifying the constellation of symptoms narrows the differential diagnosis to late-pregnancy hepatic complications.

Evaluate the laboratory data.

Significant findings include marked transaminitis (AST/ALT

> 500\text{ U/L}

), thrombocytopenia, acute kidney injury, hyperbilirubinemia, and profound hypoglycemia (

42\text{ mg/dL}

Lab values differentiate between overlapping obstetric emergencies.

Differentiate the diagnosis from HELLP syndrome.

While HELLP shares features like RUQ pain and thrombocytopenia, the profound hypoglycemia and clinical encephalopathy point toward primary liver synthetic failure, which is diagnostic of Acute Fatty Liver of Pregnancy (AFLP).

Hypoglycemia is a key distinguishing feature that indicates severe hepatocellular failure characteristic of AFLP rather than the microangiopathy of HELLP.

Identify the underlying mechanism of the diagnosed condition.

AFLP is caused by a maternal-fetal defect in fatty acid oxidation, leading to microvesicular fatty infiltration of hepatocytes.

Answering the specific question asked in the stem regarding pathophysiology.

Key Concept

Acute fatty liver of pregnancy (AFLP) is a life-threatening complication characterized by microvesicular steatosis leading to profound liver synthetic failure, distinguished from HELLP syndrome by the prominent presence of hypoglycemia, coagulopathy, and encephalopathy.

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