Nephrology Practice Questions — USMLE Step 2 CK — Page 6

Question 101Question →

A 64-year-old man is evaluated in the intensive care unit for abnormal laboratory findings. He was admitted 2 days ago for severe community-acquired pneumonia complicated by septic shock. He was intubated yesterday for worsening hypoxia and is currently mechanically ventilated. The patient is sedated and receiving a continuous intravenous infusion of norepinephrine. Vital signs show a temperature of $38.4^\circ\text{C}$ ( $101.1^\circ\text{F}$ ), blood pressure of $92/58 \text{ mm Hg}$ , heart rate of $118/\text{min}$ , and oxygen saturation of $95\%$ on the current ventilator settings.

Laboratory studies:
Serum sodium: $140 \text{ mEq/L}$
Serum potassium: $4.1 \text{ mEq/L}$
Serum chloride: $100 \text{ mEq/L}$
Serum bicarbonate: $16 \text{ mEq/L}$
Blood urea nitrogen: $32 \text{ mg/dL}$
Serum creatinine: $1.9 \text{ mg/dL}$
Serum lactate: $5.2 \text{ mmol/L}$ (Normal: $0.5-1.5 \text{ mmol/L}$ )

Arterial blood gas (on mechanical ventilation):
pH: $7.52$
$\text{PaCO}_2$ : $20 \text{ mm Hg}$
$\text{PaO}_2$ : $78 \text{ mm Hg}$

Which of the following is the most accurate interpretation of this patient's current acid-base status?

Mixed respiratory alkalosis and high anion gap metabolic acidosis

Compensated high anion gap metabolic acidosis

Primary respiratory alkalosis with appropriate metabolic compensation

Mixed respiratory alkalosis and normal anion gap metabolic acidosis

Mixed respiratory acidosis and high anion gap metabolic acidosis

Show answer & explanation

Answer: Mixed respiratory alkalosis and high anion gap metabolic acidosis

Answer

Mixed respiratory alkalosis and high anion gap metabolic acidosis

To accurately determine this patient's acid-base status, one must apply a stepwise mathematical approach. First, observe the pH: at

7.52

, the patient has alkalemia. Next, find the primary driver of this alkalemia: the

\text{PaCO}_2

is markedly low at

20 \text{ mm Hg}

, which directly causes alkalemia. This identifies a primary respiratory alkalosis, which in this ICU patient is iatrogenic due to hyperventilation by the mechanical ventilator. Crucially, acid-base analysis does not stop at the pH and

\text{PaCO}_2

; the anion gap (AG) must be calculated for every patient. The formula is

AG = \text{Na}^+ - (\text{Cl}^- + \text{HCO}_3^-)

. Here,

AG = 140 - (100 + 16) = 24 \text{ mEq/L}

. A normal AG is

8-12 \text{ mEq/L}

. An AG greater than

12 \text{ mEq/L}

definitively proves the presence of an independent, primary high anion gap metabolic acidosis (HAGMA). In this patient, the HAGMA is caused by lactic acidosis secondary to septic shock. Finally, a delta-delta ratio calculation (

12 / 8 = 1.5

) confirms there is no third simultaneous disorder (like a concurrent normal anion gap acidosis or metabolic alkalosis). Therefore, the patient has two primary, concurrent acid-base disturbances.

Step-by-Step Solution

1

Analyze the pH to determine the predominant acid-base disorder.

The pH is

7.52

(alkalemia).

Identifying whether the patient is overall acidemic or alkalemic points to the primary disorder.

2

Examine the

\text{PaCO}_2

and

\text{HCO}_3^-

to find the driver of the alkalemia.

The

\text{PaCO}_2

is

20 \text{ mm Hg}

(low).

A low

\text{PaCO}_2

causes alkalemia. Therefore, a primary respiratory alkalosis is present, likely due to excessive minute ventilation on the mechanical ventilator.

3

Calculate the serum anion gap (AG).

AG = \text{Na}^+ - (\text{Cl}^- + \text{HCO}_3^-) = 140 - (100 + 16) = 24 \text{ mEq/L}

.

An AG must always be calculated. A normal AG is

8-12 \text{ mEq/L}

. An AG of

24 \text{ mEq/L}

proves the presence of a primary high anion gap metabolic acidosis (HAGMA), driven here by elevated lactate from septic shock.

4

Evaluate the Delta-Delta ratio to check for a third disorder.

\Delta AG = 24 - 12 = 12

.

\Delta \text{HCO}_3^- = 24 - 16 = 8

. Ratio =

12 / 8 = 1.5

.

A ratio between

1

and

2

indicates an isolated HAGMA with no additional normal anion gap metabolic acidosis or metabolic alkalosis. Thus, the final diagnosis is a mixed dual disorder.

Key Concept

In mixed acid-base disorders, an alkalemic pH driven by a low

\text{PaCO}_2

defines a primary respiratory alkalosis, while an elevated anion gap definitively establishes the concurrent presence of a primary high anion gap metabolic acidosis.

Question 102Question →

A 65-year-old man is admitted to the hospital for acute decompensated heart failure and is treated with aggressive intravenous furosemide. On hospital day 3, his blood pressure is 105/65 mm Hg and heart rate is 92/min. Over the past 24 hours, his urine output has decreased to 400 mL. His serum creatinine rises from a baseline of 1.1 mg/dL to 2.8 mg/dL.

Laboratory studies are obtained to evaluate his acute kidney injury:

Serum chemistry:
Sodium: 136 mEq/L
Potassium: 3.8 mEq/L
Chloride: 98 mEq/L
Bicarbonate: 28 mEq/L
Blood urea nitrogen (BUN): 84 mg/dL
Creatinine: 2.8 mg/dL

Urine chemistry:
Sodium: 70 mEq/L
Urea nitrogen: 420 mg/dL
Creatinine: 56 mg/dL

Because recent loop diuretic administration impairs tubular sodium reabsorption, the fractional excretion of sodium ( $FE_{Na}$ ) can be misleadingly elevated. Instead, the fractional excretion of urea ( $FE_{Urea}$ ) is preferred to differentiate between prerenal azotemia and intrinsic acute tubular necrosis in this clinical scenario.

Calculate the patient's $FE_{Urea}$ . Express your answer as a percentage.

Show answer & explanation

Answer: 25

Answer

25

The correct calculation for

FE_{Urea}

is

[(420 \times 2.8) / (84 \times 56)] \times 100 = 25\%

. In the evaluation of acute kidney injury, establishing whether the etiology is prerenal (decreased perfusion) or intrinsic (tubular damage) is critical. While the fractional excretion of sodium (

FE_{Na}

) is standard, loop diuretics like furosemide block the Na-K-2Cl cotransporter in the thick ascending limb of the loop of Henle, forcibly wasting sodium into the urine. This often results in an

FE_{Na} > 1\%

(in this patient, roughly 2.6%), which falsely points toward intrinsic damage. Because urea reabsorption occurs primarily in the proximal tubule and is largely unaffected by loop diuretics,

FE_{Urea}

remains a highly sensitive and specific metric. An

FE_{Urea}

less than 35% strongly confirms a prerenal state driven by severe heart failure and aggressive diuresis.

Step-by-Step Solution

1

Identify the correct formula for the Fractional Excretion of Urea (FEUrea).

FE_{Urea} = \left( \frac{Urine_{Urea} \times Serum_{Cr}}{Serum_{BUN} \times Urine_{Cr}} \right) \times 100

This formula standardizes urea clearance against creatinine clearance to assess proximal tubular handling of urea independent of sodium.

2

Extract the relevant variables from the provided laboratory data.

Urine Urea Nitrogen = 420 mg/dL; Serum Creatinine = 2.8 mg/dL; Serum BUN = 84 mg/dL; Urine Creatinine = 56 mg/dL.

Accurate identification is required to avoid incorrectly mixing serum and urine values during calculation.

3

Calculate the numerator by multiplying Urine Urea by Serum Creatinine.

420 \times 2.8 = 1176

This represents the relative amount of urea excreted compared to the serum creatinine level.

4

Calculate the denominator by multiplying Serum BUN by Urine Creatinine.

84 \times 56 = 4704

This represents the filtered load based on serum BUN normalized by the urine creatinine level.

5

Divide the numerator by the denominator and multiply by 100.

(1176 / 4704) \times 100 = 0.25 \times 100 = 25\%

This calculation yields the final percentage of filtered urea that is actually excreted by the kidneys.

Key Concept

Fractional Excretion of Urea (FEUrea) is a more reliable marker than FENa for differentiating prerenal azotemia from intrinsic acute tubular necrosis in patients actively receiving loop diuretics.

Question 103Question →

Match each of the following patient presentations, which are characterized by specific acid-base and electrolyte derangements (Left), with the precise underlying cellular or transporter defect (Right).

Click a left item, then click its matching right item

Show answer & explanation

Answer

The correct matches align the hypocalciuric normotensive patient with the DCT NCCT defect, the hypertensive patient with suppressed aldosterone to ENaC hyperactivity, the patient with nephrocalcinosis and alkaline urine to impaired alpha-intercalated cell H+ secretion, and the multiple myeloma patient with normoglycemic glycosuria to proximal convoluted tubule dysfunction.

Arriving at the correct diagnosis for each tubulopathy requires synthesizing the acid-base status, serum electrolytes, blood pressure, and specific urinary findings (calcium excretion, glucose, pH). Once the clinical syndrome is accurately identified, it maps precisely to a unique anatomical location and molecular transporter defect within the nephron.

Step-by-Step Solution

1

Evaluate the patient presenting with normotension, hypokalemia, metabolic alkalosis, and hypocalciuria.

Identify Gitelman syndrome, which maps to a defect in the distal convoluted tubule (NCCT).

The unique combination of salt-wasting (hypokalemia, alkalosis) with normotension and low urine calcium uniquely points to a distal convoluted tubule defect rather than a loop of Henle defect (Bartter syndrome).

2

Evaluate the patient with severe hypertension, hypokalemic metabolic alkalosis, and suppressed renin/aldosterone.

Identify Liddle syndrome, which maps to a gain-of-function mutation in ENaC.

If aldosterone is suppressed despite features of hyperaldosteronism, the defect must be constitutive activation of the downstream sodium channel in the collecting duct.

3

Evaluate the patient with a non-anion gap metabolic acidosis, hypokalemia, nephrocalcinosis, and fasting urine pH of 6.8.

Identify Type 1 (distal) RTA, which maps to defective H+ secretion by alpha-intercalated cells.

The inability to acidify urine below a pH of 5.5, combined with calcium phosphate stones, localizes the defect precisely to the distal nephron's proton pumps.

4

Evaluate the patient with multiple myeloma presenting with non-anion gap metabolic acidosis, hypokalemia, and normoglycemic glycosuria.

Identify Fanconi syndrome (Type 2 proximal RTA), which maps to generalized proximal convoluted tubule dysfunction.

Glycosuria in the absence of hyperglycemia implies a failure of the proximal tubule's Na+/glucose cotransporter, which often accompanies a failure to reabsorb bicarbonate in Fanconi syndrome.

Key Concept

Pathophysiology and localization of renal tubular defects, including renal tubular acidoses and inherited tubulopathies.

Estimated Time:3m 0s

Question 104Question →

A 45-year-old man with a history of poorly controlled type 2 diabetes mellitus is brought to the emergency department due to confusion, nausea, and abdominal pain for the past 2 days. He has missed his last several doses of insulin. His temperature is 37.1°C (98.8°F), blood pressure is 100/65 mm Hg, pulse is 112/min, and respirations are 24/min. Physical examination shows dry mucous membranes and a fruity breath odor.

Laboratory studies show:

Laboratory Test	Result
Sodium	142 mEq/L
Potassium	4.8 mEq/L
Chloride	102 mEq/L
Bicarbonate	14 mEq/L
Blood urea nitrogen	25 mg/dL
Creatinine	1.4 mg/dL
Glucose	450 mg/dL

What is this patient's serum anion gap (in mEq/L)?

Show answer & explanation

Answer: 26

Answer

26

The serum anion gap is calculated using the formula:

\text{Anion Gap} = \text{Na}^+ - (\text{Cl}^- + \text{HCO}_3^-)

. Using the patient's laboratory values, the calculation is

142 - (102 + 14) = 142 - 116 = 26

mEq/L. A normal anion gap is typically between 8 and 12 mEq/L. This patient has an elevated anion gap of 26 mEq/L, consistent with high anion gap metabolic acidosis secondary to diabetic ketoacidosis.

Step-by-Step Solution

1

Identify the formula for serum anion gap.

\text{Anion Gap} = \text{Na}^+ - (\text{Cl}^- + \text{HCO}_3^-)

This is the standard formula used to evaluate the etiology of metabolic acidosis.

2

Extract the necessary serum electrolyte values from the vignette.

\text{Na}^+ = 142

,

\text{Cl}^- = 102

,

\text{HCO}_3^- = 14

.

These are the three variables required for the calculation.

3

Perform the calculation.

142 - (102 + 14) = 142 - 116 = 26

.

To determine the exact numerical value of the anion gap.

Key Concept

Calculation of the serum anion gap in the evaluation of metabolic acidosis

Question 105Question →

Match each classic urinalysis sediment finding with its most likely associated renal pathology.

Click a left item, then click its matching right item

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Answer

Muddy brown granular casts match with Acute tubular necrosis. Red blood cell casts match with Acute glomerulonephritis. White blood cell casts match with Acute interstitial nephritis. Fatty casts match with Nephrotic syndrome. Broad, waxy casts match with Advanced chronic kidney disease.

Each type of urinary cast has a specific mechanism of formation that correlates with a distinct pathophysiology. Muddy brown casts reflect tubular necrosis. RBC casts indicate glomerular basement membrane injury. WBC casts indicate tubulointerstitial inflammation. Fatty casts indicate lipiduria from nephrotic syndrome. Broad waxy casts indicate dilated tubules in chronic kidney disease.

Step-by-Step Solution

1

Identify the etiology of muddy brown granular casts.

Associated with acute tubular necrosis (ATN).

Ischemia or nephrotoxins cause necrosis of tubular epithelial cells, which slough off and form these characteristic casts.

2

Identify the etiology of red blood cell (RBC) casts.

Associated with acute glomerulonephritis.

RBCs escape through a damaged glomerular basement membrane and get trapped in Tamm-Horsfall mucoprotein in the tubules.

3

Identify the etiology of white blood cell (WBC) casts.

Associated with acute interstitial nephritis or pyelonephritis.

Inflammation of the interstitium and tubules leads to WBC exudation and cast formation.

4

Identify the etiology of fatty casts.

Associated with nephrotic syndrome.

Massive proteinuria stimulates hepatic lipoprotein synthesis, leading to hyperlipidemia and lipiduria, forming fatty casts.

5

Identify the etiology of broad, waxy casts.

Associated with advanced chronic kidney disease.

Chronic kidney disease leads to tubular dilation and hypertrophy; slow urine flow allows casts to form and become waxy.

Key Concept

Classic urinalysis cast findings and their specific renal etiologies

Question 106Question →

Match each set of arterial blood gas (ABG) and basic metabolic panel (BMP) results with the most likely clinical scenario. Assume all values were obtained simultaneously on room air. Reference ranges: $Na^+$ 135-145 mEq/L, $Cl^-$ 98-106 mEq/L, $HCO_3^-$ 22-28 mEq/L, $pCO_2$ 35-45 mmHg, pH 7.35-7.45.

Click a left item, then click its matching right item

Show answer & explanation

Answer

Profile 1 matches Diabetic ketoacidosis; Profile 2 matches Advanced chronic kidney disease complicated by profuse vomiting; Profile 3 matches Normal physiologic changes of third-trimester pregnancy; Profile 4 matches Severe, large-volume secretory diarrhea; and Profile 5 matches Aggressive loop diuretic therapy.

Each acid-base profile uniquely aligns with a specific clinical pathophysiology. Methodically calculating the anion gap, expected respiratory compensation, and the delta-delta ratio reveals the underlying single or mixed acid-base disorders, which are required to match the corresponding clinical scenarios accurately.

Step-by-Step Solution

1

Calculate the anion gap (

AG = Na^+ - [Cl^- + HCO_3^-]

) for each profile to rapidly differentiate gap from non-gap metabolic acidosis.

Profile 1 (

AG = 26

) and Profile 2 (

AG = 20

) have high anion gaps. Profiles 3, 4, and 5 have normal gaps (

< 12

).

Calculating the anion gap is the essential first step in evaluating any acid-base disorder, even if the pH is entirely normal.

2

Verify appropriate respiratory compensation for primary metabolic disorders using Winter's formula (

pCO_2 = 1.5 \times HCO_3^- + 8 \pm 2

) for acidosis and the alkalosis multiplier (

pCO_2 = 0.7 \times \Delta HCO_3^- + 40

).

Profile 1, 4, and 5 represent fully compensated primary metabolic disorders without a secondary respiratory process.

To rule out mixed respiratory-metabolic disorders in patients with clear primary metabolic derangements.

3

Calculate the delta-delta gap (

\Delta\Delta = [AG - 12] + measured\ HCO_3^-

) for patients with an elevated anion gap to check for hidden mixed disorders.

For Profile 2, the adjusted

HCO_3^-

is

32

mEq/L (which is

>26

), confirming a superimposed metabolic alkalosis despite a normal measured

HCO_3^-

.

A high anion gap always represents a metabolic acidosis; a normal pH and

HCO_3^-

in this setting guarantees the presence of a neutralizing metabolic alkalosis.

Key Concept

Acid-base interpretation utilizing the anion gap, expected respiratory compensation, and delta-delta gap.

Question 107Question →

On his fourth day in the surgical intensive care unit, a 48-year-old man requires reassessment due to declining urine output and worsening laboratory parameters. He originally underwent an emergent exploratory laparotomy and bowel resection for blunt abdominal trauma. On postoperative day 1, he developed severe alcohol withdrawal delirium, which has been continuously managed with a high-dose intravenous lorazepam infusion.

His temperature is $37.7^\circ\text{C}$ ( $99.9^\circ\text{F}$ ), blood pressure is $118/76\text{ mm Hg}$ , pulse is $102/\text{min}$ , and respiratory rate is $22/\text{min}$ . The abdomen is non-distended and appropriately tender at the incision site, with no rebound or guarding.

Laboratory results are as follows:

Laboratory Test	Result
Sodium ( $Na^+$ )	$140 \text{ mEq/L}$
Potassium ( $K^+$ )	$4.5 \text{ mEq/L}$
Chloride ( $Cl^-$ )	$100 \text{ mEq/L}$
Bicarbonate ( $HCO_3^-$ )	$14 \text{ mEq/L}$
Blood urea nitrogen (BUN)	$56 \text{ mg/dL}$
Creatinine	$2.8 \text{ mg/dL}$ (Baseline $0.9 \text{ mg/dL}$ )
Glucose	$180 \text{ mg/dL}$
Serum osmolality (measured)	$345 \text{ mOsm/kg H}_2\text{O}$

Arterial blood gas analysis on room air reveals:
pH: $7.28$
$PaCO_2$ : $30 \text{ mm Hg}$
$PaO_2$ : $92 \text{ mm Hg}$

Based on the provided information, which of the following is the most appropriate next step in management?

Discontinue the current sedative infusion and initiate dexmedetomidine

Administer intravenous fomepizole and prepare for hemodialysis

Obtain blood cultures and escalate to empiric broad-spectrum antibiotics

Administer a continuous intravenous sodium bicarbonate infusion

Obtain a computed tomography scan of the abdomen and pelvis with contrast

Show answer & explanation

Answer: Discontinue the current sedative infusion and initiate dexmedetomidine

Answer

Discontinue the current sedative infusion and initiate dexmedetomidine

The correct answer directly addresses the root iatrogenic cause of this patient's severe metabolic derangement. He is experiencing a high anion gap metabolic acidosis (Anion Gap = 26) coupled with a significantly elevated osmolar gap of 35 (Calculated osmolality = 310; Measured = 345). In a patient who has been hospitalized for four days, an elevated osmolar gap is virtually impossible to attribute to acute toxic alcohol ingestion (such as methanol or ethylene glycol) prior to admission, as these compounds would have long been metabolized. Instead, the continuous high-dose lorazepam infusion is the culprit. Intravenous formulations of lorazepam and diazepam utilize propylene glycol as a solvent diluent. Prolonged or high-dose infusions lead to propylene glycol accumulation, which is sequentially metabolized by alcohol dehydrogenase into lactic acid and pyruvic acid. This accumulation causes a high anion gap metabolic acidosis, hyperosmolarity, and acute tubular necrosis (evidenced by his elevated creatinine). The most critical and appropriate next step is to discontinue the offending agent (lorazepam) and switch to an alternative sedative that does not contain propylene glycol, such as dexmedetomidine, midazolam, or propofol.

Step-by-Step Solution

1

Calculate the anion gap:

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

140 - (100 + 14) = 26 \text{ mEq/L}

.

To identify the primary metabolic acid-base disorder as a High Anion Gap Metabolic Acidosis (HAGMA).

2

Evaluate respiratory compensation using Winter's formula:

PaCO_2 = 1.5 \times HCO_3^- + 8 \pm 2

1.5 \times 14 + 8 = 29 \pm 2

. The measured

PaCO_2

is

30 \text{ mm Hg}

, representing appropriate compensation.

To rule out a superimposed primary respiratory disorder.

3

Calculate the expected serum osmolality:

2 \times Na^+ + BUN/2.8 + Glucose/18

2(140) + 56/2.8 + 180/18 = 280 + 20 + 10 = 310 \text{ mOsm/kg H}_2\text{O}

.

To determine the expected osmolality for comparison against the measured laboratory value.

4

Calculate the osmolar gap: Measured Osmolality - Calculated Osmolality

345 - 310 = 35 \text{ mOsm/kg H}_2\text{O}

(Normal < 10).

A significant osmolar gap in the presence of HAGMA narrows the differential primarily to toxic alcohols or propylene glycol.

5

Synthesize the timeline and medication history

The onset of this derangement on Hospital Day 4 rules out community ingestion of toxic alcohols. The high-dose lorazepam infusion is identified as the source of propylene glycol.

To determine the iatrogenic etiology and select the most appropriate corrective intervention.

Key Concept

Propylene glycol toxicity from continuous intravenous benzodiazepine infusions (lorazepam or diazepam) presents with an acute high anion gap metabolic acidosis, a significantly elevated osmolar gap, and acute tubular necrosis.

Estimated Time:2m 30s

Question 108Question →

Paramedics are dispatched to the home of a 42-year-old man after he was discovered unresponsive in his garage. According to his husband, the patient has a history of severe depression and recently experienced a job loss. An empty, unlabeled container of sweet-smelling fluid was found nearby. Vital signs upon arrival at the hospital include a temperature of $36.5^\circ\text{C}$ ( $97.7^\circ\text{F}$ ), blood pressure of $110/70 \text{ mm Hg}$ , heart rate of $105/\text{min}$ , and respiratory rate of $24/\text{min}$ . The patient is obtunded, responding only by withdrawing from painful stimuli.

Laboratory studies are obtained:

Serum chemistry:
Sodium: $138 \text{ mEq/L}$
Potassium: $4.5 \text{ mEq/L}$
Chloride: $102 \text{ mEq/L}$
Bicarbonate: $12 \text{ mEq/L}$
Blood urea nitrogen (BUN): $28 \text{ mg/dL}$
Creatinine: $1.2 \text{ mg/dL}$
Glucose: $108 \text{ mg/dL}$

Arterial blood gas (room air):
pH: $7.22$
$\text{pCO}_2$ : $26 \text{ mm Hg}$
$\text{pO}_2$ : $95 \text{ mm Hg}$

Additional diagnostics:
Measured serum osmolality: $335 \text{ mOsm/kg H}_2\text{O}$

Based on the provided laboratory data, what is this patient's serum osmolar gap? (Enter the numerical value in $\text{mOsm/kg H}_2\text{O}$ )

Show answer & explanation

Answer: 43

Answer

43

The correct osmolar gap is 43 mOsm/kg H2O. This patient presents with a high anion gap metabolic acidosis (AG = 138 - 102 - 12 = 24 mEq/L) and an obtunded state after suspected toxic alcohol (ethylene glycol) ingestion. To determine the osmolar gap, one must first determine the calculated serum osmolality using the formula: (2 * Na) + (Glucose / 18) + (BUN / 2.8). In this patient, (2 * 138) + (108 / 18) + (28 / 2.8) = 276 + 6 + 10 = 292 mOsm/kg H2O. Subtracting this calculated value from the measured osmolality (335 mOsm/kg H2O) yields an osmolar gap of 43. A normal gap is generally less than 10; an elevated gap in this context confirms the presence of an unmeasured osmotically active substance.

Step-by-Step Solution

1

Recall the formula for calculating estimated serum osmolality in standard US units.

Calculated Osmolality = (2 \times \text{Na}) + (\text{Glucose} / 18) + (\text{BUN} / 2.8)

This formula estimates the total concentration of the major physiological solutes in the blood.

2

Input the patient's specific laboratory values into the formula.

Calculated Osmolality = (2 \times 138) + (108 / 18) + (28 / 2.8)

Sodium is 138 mEq/L, Glucose is 108 mg/dL, and BUN is 28 mg/dL.

3

Perform the calculation to find the estimated osmolality.

276 + 6 + 10 = 292 \text{ mOsm/kg H}_2\text{O}

This provides the baseline physiological osmolality without any unaccounted external osmoles.

4

Calculate the osmolar gap by subtracting the calculated osmolality from the measured osmolality.

335 - 292 = 43 \text{ mOsm/kg H}_2\text{O}

The difference reveals the presence of unmeasured osmoles, which helps diagnose toxic alcohol ingestion.

Key Concept

The serum osmolar gap is calculated by subtracting the calculated serum osmolality from the measured serum osmolality. The calculated osmolality is

(2 \times \text{Na}) + (\text{Glucose} / 18) + (\text{BUN} / 2.8)

. An elevated gap (

>10

) in the context of an anion gap metabolic acidosis strongly suggests toxic alcohol ingestion (e.g., ethylene glycol or methanol).

Question 109Question →

A 19-year-old female presents to the university clinic reporting two days of persistent nausea, subjective dizziness, and a continuous buzzing sensation in both ears. She also mentions breathing faster than normal over the last few hours. She recently had a severe viral upper respiratory infection and has been taking an over-the-counter medication for muscle aches and fever.

Vital signs are as follows:
Temperature: $37.2^\circ\text{C} (99.0^\circ\text{F})$
Blood pressure: $118/76 \text{ mmHg}$
Heart rate: $112/\text{min}$
Respiratory rate: $26/\text{min}$
Oxygen saturation: $99\%$ on room air

Laboratory studies are obtained:

Test	Result
Sodium	$141 \text{ mEq/L}$
Potassium	$3.9 \text{ mEq/L}$
Chloride	$104 \text{ mEq/L}$
Bicarbonate	$14 \text{ mEq/L}$
Blood urea nitrogen	$16 \text{ mg/dL}$
Serum creatinine	$0.8 \text{ mg/dL}$

Arterial blood gas analysis on room air reveals:

Test	Result
pH	$7.48$
$p\text{CO}_2$	$19 \text{ mmHg}$
$p\text{O}_2$	$102 \text{ mmHg}$

Which of the following best characterizes this patient's acid-base status?

Mixed primary high anion gap metabolic acidosis and primary respiratory alkalosis

Primary high anion gap metabolic acidosis with appropriate respiratory compensation

Primary respiratory alkalosis with appropriate metabolic compensation

Mixed primary non-anion gap metabolic acidosis and primary respiratory alkalosis

Primary high anion gap metabolic acidosis and primary respiratory acidosis

Show answer & explanation

Answer: Mixed primary high anion gap metabolic acidosis and primary respiratory alkalosis

Answer

The correct characterization is a mixed primary high anion gap metabolic acidosis and primary respiratory alkalosis.

The correct characterization is a mixed primary high anion gap metabolic acidosis and primary respiratory alkalosis. This patient has a classic presentation of salicylate (aspirin) toxicity, characterized by tinnitus, nausea, and hyperventilation. The acid-base analysis requires three steps: 1) The pH is

7.48

, indicating alkalemia. 2) The anion gap is

141 - (104 + 14) = 23

. Since this is

>12

, a primary high anion gap metabolic acidosis is definitively present. 3) To evaluate for a mixed disorder, Winter's formula must be applied:

\text{Expected } p\text{CO}_2 = 1.5(14) + 8 = 29 \pm 2 \text{ mmHg}

. The patient's actual

p\text{CO}_2

is

19 \text{ mmHg}

. Because the actual

p\text{CO}_2

is significantly lower than the expected compensatory range, an independent primary respiratory alkalosis is also present.

Step-by-Step Solution

1

Analyze the pH to determine the most prominent disorder.

The pH is

7.48

(

> 7.40

), indicating net alkalemia.

Establishing the net pH direction helps frame the primary physiological derangement.

2

Calculate the serum anion gap (AG).

\text{AG} = \text{Na}^+ - (\text{Cl}^- + \text{HCO}_3^-) = 141 - (104 + 14) = 23

.

An elevated AG (normal

8-12 \text{ mEq/L}

) firmly establishes the presence of a primary high anion gap metabolic acidosis, regardless of the overall pH.

3

Calculate the expected respiratory compensation using Winter's formula.

\text{Expected } p\text{CO}_2 = 1.5 \times (\text{HCO}_3^-) + 8 \pm 2 = 1.5(14) + 8 \pm 2 = 29 \pm 2 \text{ mmHg}

(Range:

27-31 \text{ mmHg}

).

Whenever a metabolic acidosis is identified, you must calculate expected respiratory compensation to rule out hidden mixed disorders.

4

Compare the actual

p\text{CO}_2

to the expected

p\text{CO}_2

.

The actual

p\text{CO}_2

is

19 \text{ mmHg}

, which is significantly lower than the expected

27-31 \text{ mmHg}

.

Because the actual

p\text{CO}_2

is driven lower than compensatory levels, the patient has an independent, concurrent primary respiratory alkalosis.

Key Concept

Patients with salicylate toxicity typically present with a mixed primary respiratory alkalosis (due to direct medullary respiratory center stimulation) and primary high anion gap metabolic acidosis (due to uncoupling of oxidative phosphorylation).

Estimated Time:1m 30s

Question 110Question →

A 38-year-old woman is referred to the nephrology clinic for evaluation of a persistent acid-base abnormality. Over the past 6 months, she has experienced progressive fatigue, generalized muscle weakness, and one episode of passing a small, radiopaque kidney stone. Her medical history is significant for severe dental caries and bilateral parotid gland enlargement, which has been present for 3 years. She complains of a constant sensation of "sand in her eyes." She takes no medications.

Vital signs are within normal limits. Physical examination shows dry mucous membranes and mildly swollen, non-tender parotid glands. Motor strength is 4/5 in the proximal lower extremities.

Laboratory studies:
$Na^+$ : $140\text{ mEq/L}$
$K^+$ : $2.6\text{ mEq/L}$
$Cl^-$ : $115\text{ mEq/L}$
$HCO_3^-$ : $12\text{ mEq/L}$
Blood urea nitrogen: $14\text{ mg/dL}$
Serum creatinine: $0.9\text{ mg/dL}$

Urinalysis:
pH: $6.5$
Specific gravity: $1.012$
Protein: Negative
Blood: Negative
Glucose: Negative
Ketones: Negative

Which of the following findings is most likely to be present in this patient?

Calcium phosphate deposition within the renal parenchyma

A negative urine anion gap

Fractional excretion of bicarbonate greater than

15\%

following alkali therapy

Decreased plasma renin activity and low serum aldosterone concentration

An arterial

pCO_2

of

32\text{ mmHg}

due to appropriate respiratory compensation

Show answer & explanation

Answer: Calcium phosphate deposition within the renal parenchyma

Answer

Calcium phosphate deposition within the renal parenchyma

This patient's presentation of dry eyes, parotid enlargement, and severe dental caries strongly suggests Sjögren syndrome. Her laboratory results reveal a normal anion gap metabolic acidosis (NAGMA) with severe hypokalemia and an inappropriately alkaline urine pH (

6.5

). This triad is highly specific for Type 1 (distal) renal tubular acidosis (RTA), which is strongly associated with autoimmune disorders like Sjögren syndrome. The primary defect in Type 1 RTA is the inability of the alpha-intercalated cells in the distal tubule to secrete hydrogen ions. This chronic acid retention leads to bone buffering (which increases urinary calcium excretion) and increased proximal tubular reabsorption of citrate (hypocitraturia). The combination of hypercalciuria, hypocitraturia, and persistently alkaline urine creates an ideal environment for calcium phosphate precipitation, resulting in radiopaque kidney stones and nephrocalcinosis (calcium deposition in the renal parenchyma).

Step-by-Step Solution

1

Identify the underlying systemic disease based on the clinical presentation.

The patient has Sjögren syndrome.

Dry eyes, dry mouth, dental caries, and parotid gland enlargement are classic signs of autoimmune exocrine gland destruction seen in Sjögren syndrome.

2

Calculate the serum anion gap to classify the metabolic acidosis.

Anion Gap =

140 - (115 + 12) = 13\text{ mEq/L}

.

An anion gap of 13 is considered normal (non-anion gap metabolic acidosis), which narrows the differential to gastrointestinal bicarbonate loss or renal tubular acidosis (RTA).

3

Analyze the potassium level and urine pH to differentiate the types of RTA.

The patient has hypokalemia (

2.6\text{ mEq/L}

) and an inappropriately alkaline urine pH (

6.5

).

In the setting of severe academia, a normal kidney should acidify the urine (pH < 5.5). The inability to do so, combined with severe hypokalemia, confirms Type 1 (distal) RTA, a known complication of Sjögren syndrome.

4

Determine the downstream complications associated with this specific renal defect.

Calcium phosphate stone formation and nephrocalcinosis.

Type 1 RTA causes chronic acidemia, which buffers protons in the bone (releasing calcium) and increases proximal citrate reabsorption. The combination of hypercalciuria, hypocitraturia, and alkaline urine leads to calcium phosphate precipitation.

Key Concept

Type 1 (Distal) Renal Tubular Acidosis Secondary to Sjögren Syndrome

Question 111Question →

Match the following clinical etiologies with their classic presentation on a serum electrolyte panel.

Click a left item, then click its matching right item

Show answer & explanation

Answer

Primary adrenal insufficiency pairs with hyponatremia/hyperkalemia/acidosis; Severe diarrhea pairs with hypokalemia/acidosis; Loop diuretic use pairs with hypokalemia/hypomagnesemia/alkalosis; Primary hyperaldosteronism pairs with hypernatremia/hypokalemia/alkalosis.

Each clinical condition alters renal tubular transport or gastrointestinal losses in specific ways that predictably change serum sodium, potassium, and acid-base balance based on their underlying mechanism of action.

Step-by-Step Solution

1

Analyze the effects of primary adrenal insufficiency on electrolytes.

Identified that lack of aldosterone causes sodium wasting, potassium retention, and acid retention.

Aldosterone normally reabsorbs sodium and secretes potassium and hydrogen ions in the collecting duct; its absence reverses this.

2

Analyze the effects of severe diarrhea on electrolytes.

Identified that diarrhea causes loss of bicarbonate and potassium.

Lower gastrointestinal secretions are rich in potassium and bicarbonate, leading to a non-anion gap acidosis and hypokalemia when lost.

3

Analyze the effects of loop diuretics on electrolytes.

Identified that they cause loss of potassium, magnesium, and hydrogen ions (alkalosis).

Inhibition of the Na-K-2Cl cotransporter reduces the lumen-positive potential, decreasing paracellular magnesium reabsorption, while increased distal delivery of sodium stimulates potassium and hydrogen secretion.

4

Analyze the effects of primary hyperaldosteronism on electrolytes.

Identified that excess aldosterone causes sodium retention, potassium loss, and hydrogen loss.

Unregulated aldosterone hypersecretion continually drives ENaC channels and apical potassium/hydrogen pumps in the distal nephron.

Key Concept

Pathophysiology of acid-base and electrolyte disturbances

Question 112Question →

Match each clinical presentation describing a patient with renal dysfunction to the most characteristic urinalysis sediment finding associated with their underlying pathology.

Click a left item, then click its matching right item

Show answer & explanation

Answer

Septic shock matches with muddy brown granular casts (ATN). Lupus nephritis matches with red blood cell casts (nephritic syndrome). Omeprazole-induced AKI matches with white blood cell casts (AIN). Diabetic nephropathy matches with oval fat bodies (nephrotic syndrome).

Urinalysis sediment provides critical clues to the etiology of renal dysfunction. Muddy brown casts represent sloughed epithelial cells in ATN. Red blood cell casts represent bleeding through an inflamed glomerulus in nephritic syndromes. White blood cell casts represent interstitial inflammation in AIN. Oval fat bodies represent lipiduria secondary to the hyperlipidemia of nephrotic syndrome.

Step-by-Step Solution

1

Analyze the first presentation: Septic shock.

Identified as ischemic injury leading to Acute Tubular Necrosis (ATN).

Hypoperfusion from shock causes ischemic damage to the metabolically active renal tubules, shedding muddy brown casts.

2

Analyze the second presentation: SLE with hematuria and hypertension.

Identified as glomerulonephritis (nephritic syndrome).

Glomerular inflammation allows red blood cells to leak into the tubules, creating dysmorphic red blood cells and red blood cell casts.

3

Analyze the third presentation: Fever, rash, and AKI after omeprazole.

Identified as Acute Interstitial Nephritis (AIN).

Drugs like proton pump inhibitors and NSAIDs can trigger an allergic hypersensitivity reaction in the renal interstitium, producing white blood cell casts and eosinophils.

4

Analyze the fourth presentation: Diabetes with massive proteinuria.

Identified as Nephrotic Syndrome.

Protein excretion over

3.5

g/day defines nephrotic syndrome. The resulting hyperlipidemia causes lipiduria, which forms oval fat bodies and fatty casts.

Key Concept

Correlation of distinct acute kidney injury and chronic kidney disease syndromes with their pathognomonic urinalysis findings.

Estimated Time:1m 30s

Question 113Question →

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 lactate

Predominant accumulation of beta-hydroxybutyrate

Accumulation of 5-oxoproline due to glutathione depletion

Intestinal loss of bicarbonate from active mucosal disease

Accumulation of unmeasured osmotically active alcohols

Show answer & explanation

Answer: Accumulation of an unmeasured stereoisomer of lactate

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

1

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.

2

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.

3

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.

4

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.

Question 114Question →

A 42-year-old woman presents to the clinic complaining of severe, generalized muscle weakness that began earlier today. She reports that she has felt increasingly fatigued over the last several months. Her past medical history is significant for recurrent kidney stones and a diagnosis of Sjögren syndrome. She currently takes no medications. Vital signs are within normal limits, except for a respiratory rate of $20/\text{min}$ . Physical examination reveals dry mucous membranes and marked proximal muscle weakness in the bilateral lower and upper extremities.

Laboratory studies are obtained:

Serum chemistry:
Sodium: $140 \text{ mEq/L}$
Potassium: $2.1 \text{ mEq/L}$
Chloride: $114 \text{ mEq/L}$
Bicarbonate: $14 \text{ mEq/L}$
Blood urea nitrogen: $14 \text{ mg/dL}$
Creatinine: $0.9 \text{ mg/dL}$

Arterial blood gas (room air):
pH: $7.26$
PaCO $_2$ : $29 \text{ mmHg}$
PaO $_2$ : $98 \text{ mmHg}$

Urinalysis:
pH: $6.8$
Specific gravity: $1.012$
Protein: Negative
Glucose: Negative

Which of the following is the most likely primary mechanism responsible for this patient's clinical and laboratory findings?

Inability of the alpha-intercalated cells to secrete hydrogen ions

Decreased capacity of the proximal tubule to reabsorb bicarbonate

Accumulation of unmeasured endogenous organic acids

Resistance to mineralocorticoid action at the principal cells

Coexisting primary stimulation of the medullary respiratory center

Show answer & explanation

Answer: Inability of the alpha-intercalated cells to secrete hydrogen ions

Answer

The primary mechanism is the inability of the alpha-intercalated cells to secrete hydrogen ions (Type 1 distal Renal Tubular Acidosis).

The correct answer identifies the pathophysiology of Type 1 (distal) Renal Tubular Acidosis (RTA). This patient presents with a normal anion gap metabolic acidosis (anion gap of 12) and profound hypokalemia. The key distinguishing feature is her inappropriately high urine pH (6.8), which indicates an inability of the distal tubule's alpha-intercalated cells to secrete hydrogen ions into the urine. This alkaline environment, combined with hypocitraturia, strongly predisposes patients to calcium phosphate kidney stones. Furthermore, Type 1 RTA is highly associated with autoimmune conditions, classically Sjögren syndrome and rheumatoid arthritis.

Step-by-Step Solution

1

Calculate the serum anion gap to classify the metabolic acidosis.

Anion Gap =

\text{Na}^+ - (\text{Cl}^- + \text{HCO}_3^-) = 140 - (114 + 14) = 12 \text{ mEq/L}

. This is a normal anion gap (non-anion gap) metabolic acidosis.

Differentiating between normal and elevated anion gap metabolic acidosis is the crucial first step in identifying the etiology.

2

Apply Winter's formula to determine if respiratory compensation is appropriate.

Expected

\text{PaCO}_2 = (1.5 \times \text{HCO}_3^-) + 8 \pm 2 = (1.5 \times 14) + 8 = 29 \text{ mmHg}

. The measured

\text{PaCO}_2

is

29 \text{ mmHg}

, indicating perfect respiratory compensation and ruling out a mixed disorder.

Ensures there is no superimposed primary respiratory alkalosis or acidosis.

3

Evaluate the potassium level, urine pH, and clinical history to determine the cause of the non-anion gap metabolic acidosis.

The patient has severe hypokalemia (

2.1 \text{ mEq/L}

), an inappropriately alkaline urine (pH

6.8

), a history of nephrolithiasis, and Sjögren syndrome.

This specific combination of findings differentiates Type 1 (distal) RTA from Type 2 (proximal) and Type 4 RTAs.

Key Concept

Type 1 (distal) renal tubular acidosis is caused by impaired hydrogen ion secretion by alpha-intercalated cells, resulting in a normal anion gap metabolic acidosis, severe hypokalemia, an inappropriately alkaline urine pH (

> 5.5

), and an increased risk of calcium phosphate kidney stones. It is strongly associated with autoimmune diseases like Sjögren syndrome.

Estimated Time:2m 0s

Question 115Question →

A 42-year-old man is brought to the emergency department by his partner due to profound weakness and lethargy. For the past four days, he has experienced profuse, watery diarrhea with up to 10 large bowel movements daily. He has not been able to tolerate oral intake. His temperature is $37.2^\circ\text{C}$ ( $99.0^\circ\text{F}$ ), pulse is $112/\text{min}$ , respirations are $22/\text{min}$ , and blood pressure is $98/64\text{ mm Hg}$ . Physical examination reveals dry mucous membranes, flat neck veins, and decreased skin turgor. Neurologic examination shows generalized muscle weakness with strength rated $3/5$ in both upper and lower extremities. Laboratory studies show the following:

Test	Result
Serum sodium	$134 \text{ mEq/L}$
Serum potassium	$2.4 \text{ mEq/L}$
Serum chloride	$114 \text{ mEq/L}$
Serum bicarbonate	$10 \text{ mEq/L}$
Blood urea nitrogen	$42 \text{ mg/dL}$
Serum creatinine	$1.8 \text{ mg/dL}$

Arterial blood gas analysis on room air reveals:

Test	Result
pH	$7.22$
pCO $_2$	$32 \text{ mm Hg}$
pO $_2$	$94 \text{ mm Hg}$

Based on the laboratory findings, which of the following is the most accurate characterization of this patient's acid-base status?

Normal anion gap metabolic acidosis and concurrent respiratory acidosis

Normal anion gap metabolic acidosis with appropriate respiratory compensation

Normal anion gap metabolic acidosis and concurrent respiratory alkalosis

High anion gap metabolic acidosis with appropriate respiratory compensation

High anion gap metabolic acidosis and concurrent respiratory acidosis

Show answer & explanation

Answer: Normal anion gap metabolic acidosis and concurrent respiratory acidosis

Answer

The patient has a normal anion gap metabolic acidosis due to gastrointestinal bicarbonate loss, complicated by a concurrent respiratory acidosis, likely secondary to hypokalemia-induced respiratory muscle weakness.

The correct answer accurately synthesizes the three necessary calculations for this patient. First, the low pH (

7.22

) and low bicarbonate (

10 \text{ mEq/L}

) establish a primary metabolic acidosis. Second, the anion gap is normal at

10 \text{ mEq/L}

(

134 - [114 + 10]

), which is consistent with bicarbonate loss from severe diarrhea. Third, Winter's formula dictates that the expected pCO2 should be

(1.5 \times 10) + 8 = 23 \pm 2 \text{ mm Hg}

. The patient's actual pCO2 is

32 \text{ mm Hg}

. Because

32

is greater than the expected maximum of

25

, the patient is inadequately ventilating to compensate for the acidosis, confirming a concurrent respiratory acidosis. This patient's profound hypokalemia (

2.4 \text{ mEq/L}

) has led to skeletal muscle weakness, which often includes the muscles of respiration, blunting the compensatory hyperventilation.

Step-by-Step Solution

1

Identify the primary acid-base disturbance.

The pH is

7.22

(acidemia) and serum

\text{HCO}_3^-

is

10 \text{ mEq/L}

(low).

Low pH with low bicarbonate confirms a primary metabolic acidosis.

2

Calculate the serum anion gap.

\text{Anion Gap} = \text{Na}^+ - (\text{Cl}^- + \text{HCO}_3^-) = 134 - (114 + 10) = 134 - 124 = 10 \text{ mEq/L}

.

An anion gap of

10 \text{ mEq/L}

is within the normal range (

8-12 \text{ mEq/L}

), identifying the primary disorder as a normal anion gap metabolic acidosis (NAGMA), typical of severe diarrhea.

3

Calculate the expected respiratory compensation using Winter's formula.

\text{Expected pCO}_2 = (1.5 \times \text{HCO}_3^-) + 8 \pm 2 = (1.5 \times 10) + 8 \pm 2 = 15 + 8 \pm 2 = 23 \pm 2 \text{ mm Hg}

.

Winter's formula determines the expected pCO2 to see if the respiratory response to the metabolic acidosis is appropriate.

4

Compare the expected pCO2 to the measured pCO2.

The measured pCO2 is

32 \text{ mm Hg}

, which is significantly higher than the expected range of

21-25 \text{ mm Hg}

.

Because the measured pCO2 is higher than expected, there is relative hypoventilation, diagnosing a superimposed respiratory acidosis (likely due to severe hypokalemia causing respiratory muscle weakness).

Key Concept

In patients with severe metabolic acidosis, the actual pCO2 must be compared to the expected pCO2 calculated by Winter's formula. If the measured pCO2 is higher than the expected value, a concurrent respiratory acidosis is present, which may be caused by hypokalemia-induced respiratory muscle fatigue.

Question 116Question →

Match each clinical vignette describing a patient with a renal disorder to the most characteristic histopathologic or immunofluorescent finding expected on their renal biopsy.

Click a left item, then click its matching right item

Show answer & explanation

Answer

The correct pairings match Granulomatosis with polyangiitis to pauci-immune crescents, Antiphospholipid syndrome TMA to fibrin thrombi, Rheumatoid arthritis-induced amyloidosis to Congo red positivity, IgA nephropathy to mesangial IgA, and Membranous nephropathy (with renal vein thrombosis) to 'spike and dome' subepithelial deposits.

Each clinical vignette provides distinct clues to a specific nephropathy. Granulomatosis with polyangiitis presents with systemic vasculitis (sinusitis) and PR3-ANCA, correlating with pauci-immune crescents. Antiphospholipid syndrome can trigger a secondary thrombotic microangiopathy, characterized by fibrin thrombi and schistocytes without immune complexes. Chronic inflammation from rheumatoid arthritis causes AA amyloidosis, definitively diagnosed via Congo red stain. IgA nephropathy uniquely presents with synpharyngitic hematuria and mesangial IgA deposits. Lastly, membranous nephropathy is heavily associated with solid malignancies and has a high propensity for renal vein thrombosis (causing sudden flank pain and a new left varicocele), showing subepithelial 'spike and dome' deposits on biopsy.

Step-by-Step Solution

1

Analyze each patient's clinical history, physical exam, and laboratory findings to establish the primary clinical diagnosis.

Identified Granulomatosis with polyangiitis (GPA), Antiphospholipid syndrome (APS) with thrombotic microangiopathy (TMA), Secondary AA Amyloidosis, IgA Nephropathy, and Membranous Nephropathy with renal vein thrombosis.

A correct clinical diagnosis is required to deduce the corresponding histopathology, as biopsy findings are pathognomonic for these syndromes.

2

Correlate each clinical diagnosis with its specific biopsy findings on light microscopy, immunofluorescence, and electron microscopy.

Matched pauci-immune crescents to GPA, fibrin thrombi to TMA, Congo red positivity to amyloidosis, mesangial IgA to IgA nephropathy, and subepithelial 'spike and dome' deposits to membranous nephropathy.

Renal pathology is defined by specific microscopic and immunologic patterns that confirm the clinical suspicion and dictate further management.

Key Concept

Correlation of complex clinical nephrology syndromes and their associated complications with pathognomonic renal biopsy findings.

Estimated Time:3m 0s

Question 117Question →

Following an emergency exploratory laparotomy for ischemic colitis, a $68$ -year-old patient remains mechanically ventilated in the surgical intensive care unit. Over the past $48$ hours, continuous nasogastric suctioning has yielded $2.5$ liters of fluid. The patient is currently receiving continuous intravenous infusions of fentanyl and propofol. Current vital signs include a temperature of $38.9^\circ C$ ( $102.0^\circ F$ ), pulse of $118$ /min, and blood pressure of $88/54$ mm Hg. The ventilator is set to synchronized intermittent mandatory ventilation mode, and the patient is triggering very few spontaneous breaths.

Laboratory evaluation demonstrates:

Parameter	Value
Arterial $pH$	$7.23$
Arterial $pCO_2$	$50$ mm Hg
Arterial $pO_2$	$72$ mm Hg
Serum $Na^+$	$140$ mEq/L
Serum $K^+$	$3.1$ mEq/L
Serum $Cl^-$	$88$ mEq/L
Serum $HCO_3^-$	$20$ mEq/L
Blood Urea Nitrogen	$42$ mg/dL
Serum Creatinine	$2.5$ mg/dL
Serum Lactate	$5.8$ mmol/L

What is the complete characterization of this patient's acid-base status?

High anion gap metabolic acidosis, metabolic alkalosis, and respiratory acidosis

High anion gap metabolic acidosis, normal anion gap metabolic acidosis, and respiratory acidosis

High anion gap metabolic acidosis and metabolic alkalosis with appropriate respiratory compensation

High anion gap metabolic acidosis and respiratory acidosis

Normal anion gap metabolic acidosis and respiratory acidosis

Show answer & explanation

Answer: High anion gap metabolic acidosis, metabolic alkalosis, and respiratory acidosis

Answer

The patient has a triple acid-base disorder consisting of a high anion gap metabolic acidosis, a metabolic alkalosis, and a respiratory acidosis.

The correct answer identifies all three components of the patient's complex acid-base disturbance. The patient has acidemia (

pH < 7.35

). Step 1: The anion gap is

32

(

140 - (88 + 20)

), indicating a high anion gap metabolic acidosis. Step 2: The delta gap is

20

(

32 - 12

). The expected bicarbonate is

4

mEq/L (

24 - 20

), but the measured bicarbonate is

20

mEq/L. This gap indicates a concurrent metabolic alkalosis. Step 3: Winter's formula predicts an expected

pCO_2

of

38 \pm 2

mm Hg (

1.5 \times 20 + 8

). The actual

pCO_2

is

50

mm Hg, which is higher than expected, confirming an additional respiratory acidosis.

Step-by-Step Solution

1

Calculate the anion gap to identify the primary metabolic disturbance.

AG = Na^+ - (Cl^- + HCO_3^-) = 140 - (88 + 20) = 32

mEq/L.

An elevated anion gap (

>12

mEq/L) indicates a primary high anion gap metabolic acidosis (HAGMA), which in this hypotensive patient is secondary to lactic acidosis and uremia.

2

Calculate the delta-delta gap to evaluate for concurrent metabolic disorders.

\Delta AG = 32 - 12 = 20

mEq/L. Expected

HCO_3^- = 24 - 20 = 4

mEq/L. The actual

HCO_3^-

is

20

mEq/L.

Because the actual

HCO_3^-

(

20

mEq/L) is significantly higher than the expected

HCO_3^-

(

4

mEq/L), there must be a concurrent process adding bicarbonate. This indicates a superimposed metabolic alkalosis, driven by the continuous nasogastric suctioning (loss of

H^+

and

Cl^-

).

3

Use Winter's formula to determine if respiratory compensation is appropriate.

Expected

pCO_2 = (1.5 \times 20) + 8 \pm 2 = 38 \pm 2

mm Hg. The actual

pCO_2

is

50

mm Hg.

Because the actual

pCO_2

(

50

mm Hg) is greater than the expected range (

36-40

mm Hg), the patient is hypoventilating relative to their metabolic state. This indicates a concurrent respiratory acidosis, secondary to heavy sedation and the set ventilator mode.

Key Concept

A triple acid-base disorder can be correctly diagnosed by sequentially evaluating the arterial pH, calculating the anion gap, utilizing the delta-delta calculation, and applying Winter's formula to assess respiratory compensation.

Estimated Time:2m 30s

Question 118Question →

Match each patient's clinical presentation with the classic renal biopsy finding most likely to be seen in their condition.

Click a left item, then click its matching right item

Show answer & explanation

Answer

DPGN corresponds to wire loops; MPGN corresponds to tram-track appearance; FSGS corresponds to segmental sclerosis; Membranous nephropathy corresponds to spike and dome pattern; Alport syndrome corresponds to basket-weave appearance.

Each vignette represents a classic systemic trigger for a specific glomerular disease. By recognizing the associations (SLE with DPGN, HCV with MPGN, HIV with FSGS, solid tumor with Membranous Nephropathy, and type IV collagen defect with Alport syndrome), one can definitively pair the clinical presentation with its pathognomonic microscopic description.

Step-by-Step Solution

1

Analyze the SLE vignette (left_1).

Identifies Diffuse Proliferative Glomerulonephritis (DPGN).

SLE with nephritic syndrome and active urinary sediment points to DPGN, which matches with 'wire loop' subendothelial deposits.

2

Analyze the Hepatitis C and cryoglobulinemia vignette (left_2).

Identifies Membranoproliferative Glomerulonephritis (MPGN) Type 1.

Chronic HCV and cryoglobulinemia are strong triggers for MPGN, matching with mesangial interposition and the 'tram-track' appearance.

3

Analyze the HIV infection vignette (left_3).

Identifies Focal Segmental Glomerulosclerosis (FSGS).

Uncontrolled HIV is classically associated with the collapsing variant of FSGS, which presents with massive proteinuria and segmental sclerosis.

4

Analyze the colon cancer vignette (left_4).

Identifies Membranous Nephropathy.

Solid organ malignancies can precipitate secondary Membranous Nephropathy, matching the subepithelial 'spike and dome' appearance.

5

Analyze the familial deafness and lenticonus vignette (left_5).

Identifies Alport Syndrome.

The clinical triad is pathognomonic for Alport syndrome, caused by a type IV collagen defect, matching the 'basket-weave' GBM finding.

Key Concept

Correlation of systemic diseases with secondary glomerular pathologies and their classic microscopic findings.

Estimated Time:2m 0s

Question 119Question →

A 68-year-old man with a history of type 2 diabetes mellitus is brought to the emergency department due to progressively worsening confusion and profound weakness over the past 4 days. He recently recovered from a mild upper respiratory illness. On examination, he is lethargic but arousable. Vital signs show a blood pressure of $92/58\text{ mm Hg}$ , a heart rate of $118/\text{min}$ , and a respiratory rate of $16/\text{min}$ . His mucous membranes are parched, and skin turgor is significantly decreased.

Initial laboratory evaluation reveals:
Serum sodium: $129\text{ mEq/L}$
Serum potassium: $4.4\text{ mEq/L}$
Serum chloride: $95\text{ mEq/L}$
Serum bicarbonate: $22\text{ mEq/L}$
Blood urea nitrogen: $48\text{ mg/dL}$
Serum creatinine: $1.9\text{ mg/dL}$
Serum glucose: $600\text{ mg/dL}$

In order to properly select the composition of intravenous fluids for resuscitation, the patient's true sodium status must be evaluated.

What is the patient's corrected serum sodium concentration in $\text{mEq/L}$ ? (Assume a correction factor of $1.6\text{ mEq/L}$ for each $100\text{ mg/dL}$ increase in glucose above $100\text{ mg/dL}$ ).

Show answer & explanation

Answer: 137

Answer

137

In the setting of severe hyperglycemia, the high concentration of glucose in the extracellular fluid creates an osmotic gradient that draws water out of cells. This transcellular fluid shift dilutes the serum sodium concentration, leading to a phenomenon known as translocational hyponatremia. To determine the patient's true sodium status, a correction factor must be applied. The standard correction adds

1.6\text{ mEq/L}

to the measured sodium for every

100\text{ mg/dL}

increase in glucose above a baseline of

100\text{ mg/dL}

. For a glucose level of

600\text{ mg/dL}

, the excess is

500\text{ mg/dL}

. Multiplying

5

by

1.6

yields a correction of

8\text{ mEq/L}

. Adding this to the measured sodium of

129\text{ mEq/L}

results in a corrected serum sodium of

137\text{ mEq/L}

. This indicates that the patient actually has a normal serum sodium concentration, which is critical for choosing the appropriate intravenous fluid for resuscitation.

Step-by-Step Solution

1

Identify the patient's measured serum sodium and serum glucose levels.

Measured Na =

129\text{ mEq/L}

, Glucose =

600\text{ mg/dL}

.

These are the two variables required to calculate the corrected serum sodium.

2

Calculate the amount of glucose elevation above the normal baseline of

100\text{ mg/dL}

.

600 - 100 = 500\text{ mg/dL}

above normal.

The correction factor is only applied to the excess glucose driving the osmotic shift.

3

Determine the total sodium correction value by applying the given factor.

Correction =

5 \times 1.6 = 8\text{ mEq/L}

.

For every

100\text{ mg/dL}

of excess glucose, sodium is artificially lowered by

1.6\text{ mEq/L}

.

4

Add the correction value to the measured serum sodium.

129 + 8 = 137\text{ mEq/L}

.

This yields the true serum sodium concentration, guiding appropriate fluid therapy.

Key Concept

Pseudohyponatremia due to hyperglycemia (translocational hyponatremia)

Question 120Question →

A 72-year-old man is brought to the emergency department by his family due to confusion and severe weakness. He has a history of severe chronic obstructive pulmonary disease (COPD) and heart failure with preserved ejection fraction. For the past three days, he has experienced a worsening fever, productive cough, and profuse, intractable vomiting, leaving him unable to tolerate oral intake. His home medications include furosemide, lisinopril, and inhaled tiotropium.

On presentation, his temperature is 38.6°C (101.5°F), blood pressure is 82/50 mm Hg, pulse is 118/min, and respiratory rate is 26/min. Physical examination reveals lethargy, dry mucous membranes, and delayed capillary refill. Chest auscultation reveals diffuse wheezing and bilateral coarse crackles.

Laboratory studies show:

Parameter	Value
Sodium	$140$ mEq/L
Potassium	$2.8$ mEq/L
Chloride	$82$ mEq/L
Bicarbonate	$18$ mEq/L
Blood urea nitrogen	$68$ mg/dL
Creatinine	$3.4$ mg/dL (baseline $1.1$ )
Lactate	$8.5$ mmol/L

Arterial blood gas on room air:

Parameter	Value
pH	$7.12$
pCO $_2$	$58$ mm Hg
pO $_2$	$54$ mm Hg

Based on the laboratory findings, which of the following combinations best describes this patient's current acid-base disturbances?

High anion gap metabolic acidosis, metabolic alkalosis, and respiratory acidosis

High anion gap metabolic acidosis and respiratory acidosis only

High anion gap metabolic acidosis, normal anion gap metabolic acidosis, and respiratory acidosis

High anion gap metabolic acidosis and metabolic alkalosis only

Normal anion gap metabolic acidosis and respiratory acidosis only

Show answer & explanation

Answer: High anion gap metabolic acidosis, metabolic alkalosis, and respiratory acidosis

Answer

The patient has a triple acid-base disorder consisting of a high anion gap metabolic acidosis, a metabolic alkalosis, and a respiratory acidosis.

This patient presents with a highly complex clinical picture resulting in a triple acid-base disorder. First, calculating the anion gap yields

140 - (82 + 18) = 40

. An elevated gap (

> 12

) confirms a High Anion Gap Metabolic Acidosis (HAGMA), which correlates with his severe lactic acidosis (

8.5

mmol/L) and uremia from acute kidney injury. Second, to determine if another metabolic process is present, the delta gap is calculated:

\Delta AG = 40 - 12 = 28

. Adding this to the measured bicarbonate gives a 'corrected' bicarbonate of

18 + 28 = 46

mEq/L. Because this is much higher than the normal

24

mEq/L, there is a concurrent Metabolic Alkalosis, which is clinically consistent with his history of intractable vomiting and furosemide use (evidenced by severe hypochloremia and hypokalemia). Finally, Winter's formula (

1.5 \times 18 + 8 = 35

) dictates that the expected

pCO_2

should be between

33

and

37

mm Hg. The patient's actual

pCO_2

is

58

mm Hg, establishing an independent Respiratory Acidosis driven by a severe COPD exacerbation and impending respiratory failure.

Step-by-Step Solution

1

Calculate the Anion Gap (AG).

AG = Na^+ - (Cl^- + HCO_3^-) = 140 - (82 + 18) = 40

.

An

AG > 12

establishes the presence of a High Anion Gap Metabolic Acidosis (HAGMA), driven here by lactic acidosis and acute kidney injury.

2

Calculate the Delta Gap and Corrected Bicarbonate to check for a concurrent metabolic disorder.

\Delta AG = 40 - 12 = 28

. Corrected

HCO_3^- = Measured HCO_3^- + \Delta AG = 18 + 28 = 46

mEq/L.

Because the corrected bicarbonate (

46

mEq/L) is significantly greater than the normal value of

24

mEq/L, a concurrent primary Metabolic Alkalosis is present, caused by severe vomiting and furosemide use.

3

Calculate the expected

pCO_2

using Winter's formula to evaluate respiratory compensation.

Expected

pCO_2 = (1.5 \times 18) + 8 \pm 2 = 35 \pm 2

mm Hg.

To determine if the respiratory system is appropriately compensating for the measured bicarbonate level of

18

mEq/L.

4

Compare the expected

pCO_2

with the measured

pCO_2

.

Measured

pCO_2

(

58

mm Hg) > Expected

pCO_2

(

33-37

mm Hg).

Since the actual

pCO_2

is higher than expected, there is a concurrent primary Respiratory Acidosis due to the patient's COPD exacerbation and respiratory fatigue.

Key Concept

Interpretation of mixed (triple) acid-base disorders requires sequential evaluation: Anion Gap, Delta-Delta calculation, and Winter's formula.

Alternative Method

Instead of the Delta Gap method, you can use the Delta Ratio (

\Delta AG / \Delta HCO_3^-

).

\Delta AG = 40 - 12 = 28

.

\Delta HCO_3^- = 24 - 18 = 6

. The Delta Ratio is

28 / 6 \approx 4.6

. A Delta Ratio

> 2.0

strongly indicates a concurrent metabolic alkalosis.

Estimated Time:2m 0s

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