A 29-year-old woman, gravida 1, para 0, comes to the clinic for a scheduled obstetric eval… — USMLE Step 2 CK

A $29$ -year-old woman, gravida $1$ , para $0$ , comes to the clinic for a scheduled obstetric evaluation at $34$ weeks gestation. Over the past few weeks, she has noticed increased shortness of breath when climbing stairs and mild swelling around her ankles by the evening. She has no headache, visual disturbances, or right upper quadrant abdominal pain. Her blood pressure is $116/76\text{ mm Hg}$ , pulse is $88/\text{min}$ , and respirations are $16/\text{min}$ . On physical examination, a soft mid-systolic murmur is audible at the left upper sternal border. The lungs are clear to auscultation bilaterally. The uterus is palpated at $34\text{ cm}$ above the pubic symphysis, and $1+$ pitting edema is present at the bilateral ankles. The physician reviews the patient's recent laboratory results. Which of the following findings in this patient represents a pathological process requiring further diagnostic workup?

A
Serum alkaline phosphatase (ALP) of $280\text{ U/L}$
B
Arterial partial pressure of carbon dioxide ( $\text{PaCO}_2$ ) of $29\text{ mm Hg}$
C
Leukocyte count of $14,800/\text{mm}^3$
D
Total serum thyroxine ( $\text{T}_4$ ) of $14.8\text{ }\mu\text{g/dL}$
Serum alanine aminotransferase (ALT) of $68\text{ U/L}$ Answer

Answer

The correct answer is the serum alanine aminotransferase (ALT) level of

68\text{ U/L}

, as transaminases should remain within non-pregnant reference ranges during a normal pregnancy.

The correct answer is the serum alanine aminotransferase (ALT) level of

68\text{ U/L}

. During normal pregnancy, maternal physiology undergoes profound alterations, including changes in standard laboratory reference ranges. While some hepatic markers like alkaline phosphatase (ALP) normally rise, hepatocellular enzyme markers such as AST and ALT, as well as biliary excretion markers like bilirubin, remain within standard non-pregnant normal limits (or slightly decrease due to hemodilution). An elevated ALT level in the third trimester is always considered pathologic and indicates hepatocellular injury. In this patient, despite the absence of right upper quadrant pain, it warrants immediate diagnostic workup for severe obstetric complications such as HELLP syndrome (Hemolysis, Elevated Liver enzymes, Low Platelets), intrahepatic cholestasis of pregnancy, or acute viral hepatitis.

Step-by-Step Solution

Evaluate the clinical presentation for signs of true pathology versus normal physiologic changes.

The patient's dyspnea, dependent edema, and systolic ejection murmur are common, benign adaptations to increased plasma volume and expanding uterine size in the third trimester.

Establishing that the clinical symptoms are benign forces the focus onto recognizing which isolated laboratory value diverges from expected pregnancy baselines.

Analyze the laboratory options against third-trimester specific reference ranges.

ALP is physiologically elevated due to placental production. PaCO2 is physiologically low due to progesterone-driven hyperventilation. Leukocytosis and elevated total T4 are also normal pregnancy adaptations.

Applying pregnancy-specific baselines is necessary to prevent misdiagnosing normal physiologic adaptations as pathological states.

Identify the laboratory finding that does not physiologically change and remains abnormal if elevated.

Serum ALT does not normally increase in pregnancy; an elevated level (

68\text{ U/L}

) represents true hepatocellular injury.

Unlike ALP, maternal transaminases (AST/ALT) are not produced by the placenta and their elevation warrants immediate investigation for conditions like HELLP syndrome or hepatitis.

Key Concept

While alkaline phosphatase normally increases in pregnancy due to placental production, transaminases (AST, ALT) and bilirubin do not. Any elevation in ALT indicates a pathologic process requiring investigation.

Estimated Time:1m 30s

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