A 45-year-old man comes to the emergency department reporting four days of frequent, large… — USMLE Step 2 CK

A 45-year-old man comes to the emergency department reporting four days of frequent, large-volume watery stools. He feels weak and lightheaded when standing. Physical examination reveals dry mucous membranes, flat neck veins, and delayed capillary refill. Laboratory results are as follows:

Test	Result
Sodium	$138$ mEq/L
Potassium	$3.2$ mEq/L
Chloride	$114$ mEq/L
Bicarbonate	$14$ mEq/L
Arterial pH	$7.28$
PaCO2	$30$ mm Hg

Based on these findings, which of the following best describes this patient's primary acid-base disorder?

Normal anion gap metabolic acidosisAnswer
B
High anion gap metabolic acidosis
C
Metabolic alkalosis
D
Primary respiratory acidosis
E
Primary respiratory alkalosis

Answer

Normal anion gap metabolic acidosis

The patient has a low pH (

7.28

) driven by a significantly decreased serum bicarbonate (

14

mEq/L), confirming a primary metabolic acidosis. The anion gap is calculated as

Na - (Cl + HCO_3)

, which equates to

138 - (114 + 14) = 10

mEq/L. Since this value is within the standard normal reference range (

8-12

mEq/L), the diagnosis is a normal anion gap metabolic acidosis. This is a classic presentation for a patient suffering from severe diarrhea, which causes profound gastrointestinal depletion of bicarbonate, ultimately leading to a compensatory retention of serum chloride (hyperchloremic metabolic acidosis).

Step-by-Step Solution

Analyze the arterial pH to determine the primary state.

The pH is

7.28

(less than

7.35

), indicating acidemia.

Identifying acidemia or alkalemia is the necessary first step in diagnosing any acid-base disorder.

Assess the serum bicarbonate and PaCO2 to identify the primary driver of the disorder.

The bicarbonate is low (

14

mEq/L), which directly aligns with the acidemic pH, indicating a primary metabolic acidosis. The low PaCO2 (

30

mm Hg) represents respiratory compensation.

Matching the direction of the abnormal pH to the corresponding metabolic (bicarbonate) or respiratory (PaCO2) parameter accurately identifies the primary disturbance.

Calculate the serum anion gap using the formula:

Na - (Cl + HCO_3)

138 - (114 + 14) = 138 - 128 = 10

mEq/L. This is within the normal range of

8-12

mEq/L.

Categorizing a confirmed metabolic acidosis as either normal or high anion gap narrows down the underlying etiology.

Key Concept

Severe diarrhea causes a normal anion gap (hyperchloremic) metabolic acidosis due to direct gastrointestinal loss of bicarbonate.

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