A 45-year-old man is brought to the emergency department by his wife due to acute-onset co… — USMLE Step 2 CK

A 45-year-old man is brought to the emergency department by his wife due to acute-onset confusion, slurred speech, and an unsteady gait. His symptoms began earlier in the day and have progressively worsened. The patient has a history of severe Crohn's disease that necessitated multiple extensive small bowel resections three years ago, leaving him with short bowel syndrome. He does not consume alcohol and has no history of substance use. His temperature is 37.1°C (98.8°F), blood pressure is 110/70 mm Hg, heart rate is 88/min, and respiratory rate is 22/min. Physical examination reveals a confused man with an ataxic gait. His lungs are clear to auscultation. The abdomen is soft with well-healed surgical scars.

Laboratory studies show:

Laboratory Test	Result
Sodium	140 mEq/L
Potassium	4.0 mEq/L
Chloride	104 mEq/L
Bicarbonate	12 mEq/L
Blood urea nitrogen	14 mg/dL
Creatinine	0.9 mg/dL
Glucose	90 mg/dL
L-lactate	1.1 mmol/L (Normal: 0.5–2.2 mmol/L)
Osmolality (measured)	290 mOsm/kg $H_2O$
Urine ketones	Negative

Arterial blood gas analysis on room air:

Parameter	Result
pH	7.24
$pCO_2$	26 mm Hg
$pO_2$	95 mm Hg

Which of the following best describes the primary underlying mechanism for this patient's current condition?

Bacterial fermentation of unabsorbed dietary carbohydratesAnswer
B
Accumulation of an organic acid secondary to global tissue hypoxia
C
Hepatic metabolism of an ingested volatile alcohol into toxic acids
D
Inadequate alveolar ventilation superimposed on an anion gap acidosis
E
Excessive gastrointestinal loss of bicarbonate-rich fluids

Answer

The correct underlying mechanism is the bacterial fermentation of unabsorbed dietary carbohydrates.

The correct answer identifies bacterial fermentation of unabsorbed dietary carbohydrates as the mechanism for D-lactic acidosis. This patient has a high anion gap metabolic acidosis (AG = 24 mEq/L) with appropriate respiratory compensation (

pCO_2

= 26 mm Hg). After ruling out toxic alcohols (normal osmolar gap), diabetic ketoacidosis (normal glucose, negative ketones), and uremia (normal BUN/Cr), the differential narrows. In a patient with short bowel syndrome presenting with acute neurologic symptoms (confusion, ataxia), D-lactic acidosis is the classic diagnosis. Unabsorbed carbohydrates reach the colon and are fermented by bacteria (e.g., Lactobacillus) into D-lactate. Humans lack the enzyme D-lactate dehydrogenase, leading to systemic accumulation. Crucially, standard laboratory assays use L-lactate dehydrogenase and only detect the L-lactate isomer, resulting in a 'normal' serum lactate report despite profound acidosis.

Step-by-Step Solution

Calculate the serum anion gap.

Anion Gap =

Na^+ - (Cl^- + HCO_3^-) = 140 - (104 + 12) = 24

mEq/L.

To determine the primary etiology of the metabolic acidosis.

Evaluate for respiratory compensation using Winter's formula.

Expected

pCO_2 = 1.5 \times [HCO_3^-] + 8 \pm 2 = 1.5 \times 12 + 8 \pm 2 = 26 \pm 2

mm Hg. Since actual

pCO_2

is 26 mm Hg, compensation is appropriate.

To rule out a concurrent mixed respiratory disorder.

Calculate the serum osmolar gap.

Calculated Osmolality =

2 \times Na^+ + \frac{\text{Glucose}}{18} + \frac{\text{BUN}}{2.8} = 280 + 5 + 5 = 290

mOsm/kg. Measured = 290 mOsm/kg. Osmolar Gap = 0.

To rule out toxic alcohol ingestion, which is a common cause of unexplained high anion gap metabolic acidosis.

Analyze the remaining MUDPILES differential given the patient's specific laboratory results and history.

Standard L-lactate is normal, glucose is normal, ketones are negative, and renal function is normal. The patient has short bowel syndrome and neurologic symptoms.

To identify D-lactic acidosis, which occurs when unabsorbed carbohydrates in short bowel syndrome are fermented by colonic bacteria into D-lactate. Standard hospital assays only detect L-lactate.

Key Concept

Patients with short bowel syndrome can develop D-lactic acidosis due to bacterial fermentation of malabsorbed carbohydrates. This presents as a high anion gap metabolic acidosis with neurologic symptoms and a falsely normal L-lactate level.

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