Match each patient's clinical presentation with their most likely expected serum and urine… — USMLE Step 2 CK

Match each patient's clinical presentation with their most likely expected serum and urine laboratory profile.

A 35-year-old woman with a history of recurrent dental caries, dry eyes, and dysphagia presenting with sudden-onset ascending flaccid paralysis.Blood pH: 7.25, Serum $K^+$ : 2.2 mEq/L, Urine pH: 6.8, Positive urine anion gap
A 58-year-old man with monoclonal gammopathy presenting with diffuse bone pain; urinalysis reveals a positive dipstick for glucose despite a normal serum glucose.Blood pH: 7.30, Serum $K^+$ : 2.7 mEq/L, Urine pH: 5.3, Fractional excretion of bicarbonate > 15%
A 65-year-old man with long-standing type 2 diabetes mellitus and chronic kidney disease who was recently started on lisinopril therapy.Blood pH: 7.32, Serum $K^+$ : 5.9 mEq/L, Urine pH: 5.2, Positive urine anion gap
A 22-year-old woman with a BMI of 18.5 kg/m^2 presenting with orthostatic hypotension, muscle cramps, and bilateral parotid gland hypertrophy.Blood pH: 7.51, Serum $K^+$ : 2.5 mEq/L, Urine pH: 6.5, Urine $Cl^-$ : 10 mEq/L
A 42-year-old man managed chronically with a mood stabilizer for bipolar disorder presenting with profound polyuria and severe dehydration.Blood pH: 7.42, Serum $Na^+$ : 154 mEq/L, Serum $K^+$ : 4.1 mEq/L, Urine osmolality: 140 mOsm/kg

Answer

The correct pairings match Sjogren syndrome to Type 1 RTA (severe hypokalemia, urine pH > 5.5), multiple myeloma to Type 2 RTA (normoglycemic glycosuria, intact distal acidification), diabetic nephropathy on ACE inhibitor to Type 4 RTA (hyperkalemia), bulimia to metabolic alkalosis (low urine chloride), and lithium use to nephrogenic diabetes insipidus (hypernatremia, very dilute urine).

Each systemic condition targets a specific segment or function of the nephron. Sjogren syndrome destroys distal intercalated cells (Type 1 RTA). Multiple myeloma light chains poison the proximal tubule (Type 2 RTA with Fanconi). Diabetic nephropathy and ACE inhibitors eliminate the aldosterone axis (Type 4 RTA). Vomiting depletes volume and chloride, stimulating profound proximal and distal reabsorption (hypochloremic alkalosis). Lithium blocks ADH action in the collecting duct (nephrogenic DI).

Step-by-Step Solution

Analyze the clinical vignettes to identify the primary systemic disease or toxic exposure.

Identified Sjogren syndrome, multiple myeloma, diabetic nephropathy with ACEi use, bulimia nervosa, and lithium toxicity.

Each of these conditions has a highly specific effect on renal tubular function and acid-base/electrolyte homeostasis.

Determine the expected acid-base and potassium derangements for each condition.

Sjogren (distal RTA 1) = hypokalemia/acidosis; Myeloma (proximal RTA 2) = hypokalemia/acidosis; Diabetes+ACEi (RTA 4) = hyperkalemia/acidosis; Bulimia = hypokalemia/alkalosis; Lithium = normal acid-base, hypernatremia.

Potassium levels and blood pH immediately differentiate RTA 4 (hyperkalemic), vomiting (alkalosis), and nephrogenic DI (pure water loss) from RTA 1 and 2.

Use specific urine chemistries to differentiate the remaining similar disorders (Type 1 vs. Type 2 RTA).

Type 1 RTA has a strict urine pH > 5.5 due to distal acidification failure. Type 2 RTA has a urine pH < 5.5 during steady state and presents with proximal tubule defects like glycosuria.

Urine pH and the presence of Fanconi syndrome components precisely distinguish the mechanisms of normal anion gap metabolic acidosis.

Key Concept

Integration of systemic disease pathophysiology with specific renal tubular defects, focusing on the differentiation of Renal Tubular Acidoses (RTAs), metabolic alkalosis generation, and renal concentrating defects.

Estimated Time:3m 0s

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