A 48-year-old woman is brought to the emergency department by her husband due to progressi… — USMLE Step 2 CK

A 48-year-old woman is brought to the emergency department by her husband due to progressive confusion, ataxia, and slurred speech that began earlier today. Her husband reports that she was feeling entirely well until this morning. Last night, they attended a celebratory dinner where she consumed a large meal consisting heavily of pasta and bread, along with a single glass of wine. Later in the evening, she took two tablets of acetaminophen for a mild headache.

Her past medical history is significant for Crohn disease, which necessitated an extensive small bowel resection two years ago. She currently takes a multivitamin but no other daily medications.

On physical examination, her temperature is $37.0^\circ\text{C}$ ( $98.6^\circ\text{F}$ ), blood pressure is $118/76$ mm Hg, pulse is $88$ /min, and respiratory rate is $22$ /min. She is disoriented to time and place. Neurologic examination reveals a broad-based gait and slurred speech, but no focal motor or sensory deficits.

Laboratory results are as follows:

Parameter	Value
Serum sodium	$140$ mEq/L
Serum potassium	$3.8$ mEq/L
Serum chloride	$105$ mEq/L
Serum bicarbonate	$12$ mEq/L
Blood urea nitrogen	$15$ mg/dL
Serum creatinine	$0.9$ mg/dL
Serum glucose	$95$ mg/dL

Arterial blood gas (on room air):

Parameter	Value
pH	$7.28$
pCO $_2$	$26$ mm Hg
pO $_2$	$95$ mm Hg

Additional laboratory studies:

Parameter	Value
Serum standard lactate (L-lactate)	$1.0$ mmol/L (Normal: $0.5 - 2.2$ )
Serum ketones (nitroprusside test)	Negative
Measured serum osmolality	$290$ mOsm/kg H $_2$ O

Urine toxicology screen is negative.

Which of the following best explains the pathogenesis of this patient's current acid-base disturbance?

Accumulation of an unmeasured stereoisomer of lactateAnswer
B
Predominant accumulation of beta-hydroxybutyrate
C
Accumulation of 5-oxoproline due to glutathione depletion
D
Intestinal loss of bicarbonate from active mucosal disease
E
Accumulation of unmeasured osmotically active alcohols

Answer

Accumulation of an unmeasured stereoisomer of lactate

The correct answer accurately identifies the underlying pathogenesis as D-lactic acidosis. This patient presents with a primary high anion gap metabolic acidosis (pH 7.28, HCO3 12, AG 23 mEq/L) and appropriate respiratory compensation. In patients with short bowel syndrome, a large carbohydrate load can escape absorption in the small intestine and reach the colon. There, commensal bacteria rapidly ferment the carbohydrates into D-lactate, which is absorbed systemically. D-lactate acts as an unmeasured anion, causing an elevated anion gap acidosis. It is neurotoxic, leading to the classic symptoms of confusion, slurred speech, and ataxia. Importantly, standard hospital laboratory assays exclusively measure L-lactate (the isomer produced by human tissue hypoperfusion). Therefore, patients with D-lactic acidosis will present with severe metabolic acidosis but a seemingly normal serum lactate level.

Step-by-Step Solution

Calculate the serum anion gap to classify the metabolic acidosis.

Anion Gap = Sodium - (Chloride + Bicarbonate) = 140 - (105 + 12) = 23 mEq/L.

An elevated anion gap (>12 mEq/L) indicates the presence of unmeasured anions (e.g., MUDPILES) rather than bicarbonate loss.

Assess respiratory compensation using Winter's formula.

Expected pCO2 = 1.5 * Bicarbonate + 8 +/- 2 = (1.5 * 12) + 8 = 26 +/- 2 mm Hg. The actual pCO2 is 26 mm Hg.

Matching expected and actual pCO2 confirms this is a simple, fully compensated high anion gap metabolic acidosis without an additional respiratory acid-base disorder.

Calculate the osmolar gap to evaluate for toxic alcohol ingestion.

Calculated osmolality = 2*[Na] + Glucose/18 + BUN/2.8 = 280 + 5.3 + 5.4 = 290.7 mOsm/kg. Measured is 290 mOsm/kg. Gap = 0.

A normal osmolar gap (<10 mOsm/kg) effectively rules out the presence of unmeasured alcohols like methanol or ethylene glycol.

Evaluate the remaining differential for a high anion gap acidosis with negative standard lactate and negative ketones.

The patient has short bowel syndrome, acute neurological symptoms, and recently consumed a high-carbohydrate meal. This fits perfectly with D-lactic acidosis.

Unabsorbed carbohydrates reach the colon and are fermented by bacteria into D-lactate. Standard laboratory assays exclusively detect L-lactate, resulting in a 'falsely normal' serum lactate reading despite profound acidosis.

Key Concept

D-lactic acidosis should be suspected in patients with short bowel syndrome who develop neurological symptoms and a high anion gap metabolic acidosis after carbohydrate ingestion, notably presenting with a normal standard serum (L-)lactate level.

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