Match each clinical vignette and laboratory profile with the most likely underlying etiolo… — USMLE Step 2 CK

Match each clinical vignette and laboratory profile with the most likely underlying etiology of the patient's acid-base disorder.

A 42-year-old woman with Sjögren syndrome presents with profound weakness. Labs: $Na^+$ $139$ mEq/L, $K^+$ $2.1$ mEq/L, $Cl^-$ $114$ mEq/L, $HCO_3^-$ $14$ mEq/L. Urine pH is $6.5$ and the urine anion gap is positive.Type 1 (distal) renal tubular acidosis
A 58-year-old man with light chain cast nephropathy presents with bone pain. Labs: $Na^+$ $140$ mEq/L, $K^+$ $2.9$ mEq/L, $Cl^-$ $112$ mEq/L, $HCO_3^-$ $16$ mEq/L. Serum glucose is $95$ mg/dL. Urinalysis shows 3+ glucose and a pH of $5.3$ .Type 2 (proximal) renal tubular acidosis
A 64-year-old man with diabetic nephropathy presents for routine follow-up. Labs: $Na^+$ $138$ mEq/L, $K^+$ $5.9$ mEq/L, $Cl^-$ $109$ mEq/L, $HCO_3^-$ $19$ mEq/L. Urine pH is $5.2$ and the urine anion gap is positive.Type 4 (hyperkalemic) renal tubular acidosis
A 29-year-old woman with an extensive ileal resection presents with fatigue. Labs: $Na^+$ $137$ mEq/L, $K^+$ $2.8$ mEq/L, $Cl^-$ $113$ mEq/L, $HCO_3^-$ $12$ mEq/L. Urine pH is $5.0$ and the urine anion gap is negative.Gastrointestinal bicarbonate loss

Cevap

Patient 1 matches with Type 1 RTA; Patient 2 matches with Type 2 RTA; Patient 3 matches with Type 4 RTA; Patient 4 matches with Gastrointestinal bicarbonate loss.

Normal anion gap metabolic acidosis is differentiated primarily by serum potassium and the urine anion gap. Hyperkalemia points to Type 4 RTA. Hypokalemia with a negative UAG points to GI loss. Hypokalemia with a positive UAG and high urine pH points to Type 1 RTA. Hypokalemia with signs of generalized proximal dysfunction (like normoglycemic glycosuria) points to Type 2 RTA.

Adım Adım Çözüm

Calculate the serum anion gap for each patient using the formula

Na^+ - (Cl^- + HCO_3^-)

to categorize the metabolic acidosis.

All four patients have an anion gap between

10

and

12

mEq/L, indicating a normal anion gap metabolic acidosis (NAGMA).

This calculation narrows the differential diagnosis to either a renal tubular acidosis (RTA) or gastrointestinal bicarbonate loss.

Evaluate the serum potassium levels for all patients.

Patients 1, 2, and 4 have hypokalemia, whereas patient 3 has hyperkalemia (

K^+

5.9

mEq/L).

Hyperkalemia in the setting of a NAGMA is highly specific for Type 4 RTA, which is caused by aldosterone deficiency or resistance. This isolates the diagnosis for patient 3.

Analyze the urine anion gap (UAG) in the hypokalemic patients.

Patient 4 has a negative UAG, while patient 1 has a positive UAG.

A negative UAG indicates that the kidneys are appropriately excreting acid (as ammonium chloride), pointing to extrarenal (gastrointestinal) bicarbonate loss, confirming patient 4's diagnosis. A positive UAG indicates impaired renal acid excretion, pointing to an RTA.

Differentiate between Type 1 and Type 2 RTA using urine pH and additional proximal tubule markers.

Patient 1 cannot acidify the urine (pH

>5.5

), indicating distal (Type 1) RTA. Patient 2 has intact distal acidification (pH

<5.5

) but shows isolated glycosuria, a sign of proximal tubule dysfunction.

The inability to secrete hydrogen ions distally confirms Type 1 RTA. Proximal tubule defects (like Fanconi syndrome) leading to bicarbonate wasting characterize Type 2 RTA.

Anahtar Kavram

Differentiation of Normal Anion Gap Metabolic Acidosis (NAGMA) using serum potassium, urine pH, and the urine anion gap.

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