Nephrology Practice Questions — USMLE Step 2 CK — Page 13

Question 241Question →

A 45-year-old male is evaluated in the surgical intensive care unit on postoperative day 3 following an extensive small bowel resection for mesenteric ischemia. Over the last 12 hours, the nursing staff reports that his urine output has decreased to $15 \text{ mL/hr}$ . He has been receiving maintenance intravenous fluids but has had large volumes of nasogastric tube output. His temperature is $37.2^\circ\text{C}$ ( $99.0^\circ\text{F}$ ), blood pressure is $90/55 \text{ mmHg}$ , and pulse is $118/\text{min}$ . Physical examination reveals flat jugular veins, delayed capillary refill, and dry mucous membranes.

Laboratory studies are obtained:

Parameter	Result
Serum sodium	$140 \text{ mEq/L}$
Serum blood urea nitrogen (BUN)	$42 \text{ mg/dL}$
Serum creatinine	$1.4 \text{ mg/dL}$
Urine sodium	$30 \text{ mEq/L}$
Urine creatinine	$60 \text{ mg/dL}$
Urine osmolality	$550 \text{ mOsm/kg}$

Based on the provided laboratory values, what is this patient's fractional excretion of sodium (FENa)? (Provide your answer as a numerical percentage, e.g., if the calculated value is 1.2%, enter 1.2)

Show answer & explanation

Answer: 0.5

Answer

0.5

The fractional excretion of sodium (FENa) measures the percentage of filtered sodium that is eventually excreted in the urine. It is calculated using the formula:

\text{FENa} = \frac{U_{Na} \times P_{Cr}}{P_{Na} \times U_{Cr}} \times 100

. Plugging in the patient's specific lab values yields

\frac{30 \times 1.4}{140 \times 60} \times 100 = \frac{42}{8400} \times 100 = 0.5\%

. A FENa of less than

1\%

indicates appropriate and robust renal conservation of sodium, strongly suggesting a prerenal etiology for the acute kidney injury (AKI). This aligns with the patient's clinical presentation of significant hypovolemia due to poor intake and high nasogastric output.

Step-by-Step Solution

1

Recall the formula for Fractional Excretion of Sodium (FENa).

\text{FENa} = \frac{U_{Na} \times P_{Cr}}{P_{Na} \times U_{Cr}} \times 100

FENa evaluates acute kidney injury by comparing the fractional clearance of sodium to the fractional clearance of creatinine, which differentiates prerenal from intrinsic renal causes.

2

Extract the necessary variables from the patient's laboratory results.

Urine Sodium (

U_{Na}

) =

30 \text{ mEq/L}

, Serum Creatinine (

P_{Cr}

) =

1.4 \text{ mg/dL}

, Serum Sodium (

P_{Na}

) =

140 \text{ mEq/L}

, Urine Creatinine (

U_{Cr}

) =

60 \text{ mg/dL}

.

Accurate identification and isolation of the four required variables avoids calculation errors.

3

Input the variables into the FENa formula and perform the calculation.

\text{FENa} = \frac{30 \times 1.4}{140 \times 60} \times 100 = \frac{42}{8400} \times 100 = 0.005 \times 100 = 0.5\%

Solving the equation yields the physiological parameter required to determine the specific etiology of the patient's oliguria.

Key Concept

The fractional excretion of sodium (FENa) is an essential diagnostic calculation used to distinguish between prerenal acute kidney injury (FENa < 1%) and intrinsic acute tubular necrosis (FENa > 2%).

Estimated Time:1m 30s

Question 242Question →

During an evening shift, a critical care team evaluates a patient who was transferred from an outside hospital following a suicide attempt. The patient was initially found stuporous by a roommate next to an empty bottle of an unknown over-the-counter medication. Prior to transfer, the patient experienced several episodes of large-volume, nonbilious emesis. Current vital signs include a temperature of $38.6^\circ\text{C}$ ( $101.5^\circ\text{F}$ ), blood pressure of $110/68$ mmHg, heart rate of $122/\text{min}$ , and respiratory rate of $34/\text{min}$ .

Laboratory results are obtained:

Arterial blood gas (room air)
pH: $7.32$
pCO $_2$ : $16$ mmHg
pO $_2$ : $98$ mmHg

Serum chemistry
Na $^+$ : $142$ mEq/L
K $^+$ : $3.4$ mEq/L
Cl $^-$ : $92$ mEq/L
HCO $_3^-$ : $12$ mEq/L
BUN: $22$ mg/dL
Creatinine: $1.3$ mg/dL

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

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

High anion gap metabolic acidosis and respiratory alkalosis

High anion gap metabolic acidosis and metabolic alkalosis

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

Normal anion gap metabolic acidosis and respiratory alkalosis

Show answer & explanation

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

Answer

The correct combination of disturbances is high anion gap metabolic acidosis, metabolic alkalosis, and respiratory alkalosis.

This patient's clinical presentation (tinnitus, fever, tachypnea, vomiting after an overdose) is highly suspicious for salicylate toxicity, which classically causes a mixed acid-base disorder. The laboratory values demonstrate a triple acid-base disturbance. First, the acidemic pH (

7.32

) with low HCO

_3^-

(

12

mEq/L) establishes a primary metabolic acidosis. Second, the anion gap is elevated at

38

(

142 - [92 + 12]

), defining a high anion gap metabolic acidosis. Third, Winter's formula (

1.5 \times 12 + 8 = 26 \pm 2

) predicts a compensatory pCO

_2

of

24-28

mmHg; the patient's actual pCO

_2

of

16

mmHg is lower than expected, revealing a concurrent respiratory alkalosis. Finally, the delta ratio (

\Delta

AG /

\Delta

HCO

_3^-

) is calculated as

(38 - 12) / (24 - 12) = 26 / 12 = 2.16

. A delta ratio greater than

2.0

signifies that a concurrent metabolic alkalosis is present, consistent with the patient's history of significant vomiting.

Step-by-Step Solution

1

Determine the primary driving disorder using pH and serum bicarbonate.

Primary metabolic acidosis.

The pH is acidemic (

7.32

) and the bicarbonate is low (

12

mEq/L).

2

Calculate the serum anion gap to determine the etiology of the metabolic acidosis.

Anion gap =

142 - (92 + 12) = 38

mEq/L. This indicates a High Anion Gap Metabolic Acidosis (HAGMA).

Differentiates between anion gap and non-anion gap metabolic acidosis.

3

Apply Winter's formula to evaluate the expected respiratory compensation.

Expected pCO

_2 = 1.5 \times 12 + 8 \pm 2 = 26 \pm 2

mmHg. The actual pCO

_2

is

16

mmHg. Because

16 < 24

, there is a concurrent primary Respiratory Alkalosis.

Determines if the patient's hyperventilation is a normal physiological response or an independent primary disorder (common in salicylate toxicity).

4

Calculate the delta-delta ratio (

\Delta

Ratio) to identify hidden concurrent metabolic disturbances.

\Delta

AG =

38 - 12 = 26

.

\Delta

HCO

_3^-

=

24 - 12 = 12

.

\Delta

Ratio =

26 / 12 = 2.16

. A ratio

> 2.0

indicates a concurrent Metabolic Alkalosis.

A ratio

> 2.0

means the increase in unmeasured anions is much larger than the drop in bicarbonate, implying that a process is simultaneously adding bicarbonate to the blood (in this case, from severe vomiting).

Key Concept

The systematic approach to complex acid-base disorders requires calculating the anion gap, applying Winter's formula to check for primary respiratory derangements, and calculating the delta ratio to uncover hidden metabolic disturbances.

Question 243Question →

A 34-year-old man is brought to the emergency department after being found unresponsive in his garage. Emergency medical personnel found a half-empty container of commercial antifreeze nearby. His blood pressure is 145/90 mm Hg, pulse is 112/min, and respirations are 24/min. Physical examination reveals deep, rapid breathing.

Laboratory studies show the following:
Serum sodium: 142 mEq/L
Serum potassium: 4.5 mEq/L
Serum chloride: 104 mEq/L
Serum bicarbonate: 12 mEq/L
Blood urea nitrogen: 28 mg/dL
Serum creatinine: 1.4 mg/dL
Serum glucose: 126 mg/dL
Measured serum osmolality: 345 mOsm/kg $H_2O$

Arterial blood gas on room air shows a pH of 7.22 and $pCO_2$ of 26 mm Hg.

Based on the provided laboratory values, what is this patient's serum osmolar gap?

Show answer & explanation

Answer: 44

Answer

The patient's calculated serum osmolar gap is 44 mOsm/kg.

The correct osmolar gap is derived by subtracting the calculated serum osmolality from the measured serum osmolality. Calculated osmolality is found using the formula:

2 \times Sodium + (BUN / 2.8) + (Glucose / 18)

. For this patient,

2(142) + (28 / 2.8) + (126 / 18) = 284 + 10 + 7 = 301

mOsm/kg. Subtracting this expected value from the measured osmolality of 345 mOsm/kg yields a gap of 44 mOsm/kg, strongly suggesting toxic alcohol (ethylene glycol) poisoning.

Step-by-Step Solution

1

Calculate the patient's expected serum osmolality using the standard formula.

Calculated Osmolality =

284 + 10 + 7 = 301

mOsm/kg.

Serum osmolality is primarily determined by sodium and its accompanying anions, urea, and glucose. The formula

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

converts US units (mg/dL) to SI units (mmol/L) to find the total expected osmoles.

2

Subtract the calculated expected osmolality from the measured osmolality provided in the laboratory results.

345 - 301 = 44

mOsm/kg.

The osmolar gap represents unmeasured osmotically active particles in the blood. A gap greater than 10-15 mOsm/kg is highly suggestive of toxic alcohol ingestion, such as ethylene glycol.

Key Concept

Diagnosis of toxic alcohol ingestion using the osmolar gap, calculated by subtracting the expected osmolality from the measured osmolality.

Question 244Question →

Evaluate the following clinical vignettes describing patients who developed acute kidney injury (AKI) in various clinical settings. Match each patient's clinical presentation to the most accurate pathophysiological diagnosis.

Click a left item, then click its matching right item

Show answer & explanation

Answer

The presentations map to distinct mechanisms: overdiuresis causes classic prerenal azotemia, cirrhosis with albumin-unresponsive AKI represents hepatorenal syndrome, post-contrast AKI is contrast-induced nephropathy (notable for FENa < 1%), and post-angiography AKI with livedo reticularis is atheroembolic renal disease.

The correct matches rely on synthesizing the patient history with specific diagnostic markers. Furosemide diuresis predictably shrinks the effective arterial blood volume, leading to a classic prerenal pattern characterized by a high BUN/Cr ratio and hyaline casts. Hepatorenal syndrome is distinguished from simple prerenal azotemia by its complete failure to improve after withdrawing diuretics and administering a dedicated volume expansion trial with intravenous albumin. Contrast-induced nephropathy represents a toxic injury to the tubules, but unlike classic ischemic acute tubular necrosis (which typically exhibits a FENa > 2%), the profound vasoconstrictive effect of the contrast media classically yields a FENa < 1%. Finally, atheroembolic renal disease is distinguished by its delayed onset (days to weeks post-procedure) alongside systemic signs of crystal embolization, such as livedo reticularis, blue toe syndrome, and eosinophilia.

Step-by-Step Solution

1

Evaluate the temporal context and inciting events for the acute kidney injury in each vignette.

Recent interventions include massive diuresis (first patient), progressive cirrhosis (second patient), iodinated contrast administration (third patient), and an invasive endovascular catheterization (fourth patient).

Establishing the precise clinical context acts as the primary branching logic in determining the etiology of acute kidney injury.

2

Analyze the accompanying physical examination signs and specific laboratory indices (urinalysis, FENa, urine sodium) for each presentation.

The first patient demonstrates a highly elevated BUN/Cr ratio with hyaline casts; the second shows fluid-unresponsiveness with severely depressed urine sodium; the third shows granular casts but a paradoxically low FENa (< 1%); the fourth displays cutaneous manifestations (livedo reticularis) and eosinophilia.

Urine indices and systemic signs separate etiologies that might otherwise present with similar temporal profiles, confirming the specific pathophysiologic mechanism.

Key Concept

Differentiating etiologies of acute kidney injury requires recognizing classic clinical contexts and appropriately interpreting urine indices, while maintaining high suspicion for exceptions such as contrast nephropathy, which typically presents with a FENa < 1%.

Estimated Time:2m 0s

Question 245Question →

Paramedics transport a 68-year-old male resident of a skilled nursing facility to the acute care clinic due to profound lethargy and weakness over the past 24 hours. According to the accompanying transfer paperwork, the patient has experienced large-volume, watery stools for five consecutive days following a treatment course of clindamycin for cellulitis. His daily medications include a transdermal fentanyl patch for severe osteoarthritis and oral acetaminophen. Initial triage vital signs show a temperature of 37.1 C (98.8 F), blood pressure of 88/58 mm Hg, heart rate of 114/min, and a respiratory rate of 9/min. Physical assessment is notable for somnolence, a delayed capillary refill of 4 seconds, and dry oral mucosa. Blood is drawn for an arterial blood gas and serum chemistry panels with the following results:

Lab Test	Value
Serum sodium	$135 \text{ mEq/L}$
Serum potassium	$2.9 \text{ mEq/L}$
Serum chloride	$112 \text{ mEq/L}$
Serum bicarbonate	$14 \text{ mEq/L}$
Blood urea nitrogen	$42 \text{ mg/dL}$
Serum creatinine	$1.9 \text{ mg/dL}$

Arterial blood gas (room air):

Parameter	Value
$pH$	$7.25$
$PaCO_2$	$35 \text{ mm Hg}$
$PaO_2$	$78 \text{ mm Hg}$

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

Normal anion gap metabolic acidosis with appropriate respiratory compensation

Normal anion gap metabolic acidosis and concurrent respiratory acidosis

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 most accurate interpretation is a normal anion gap metabolic acidosis with a concurrent respiratory acidosis.

The patient's lab results show a primary metabolic acidosis (

pH < 7.35

and low

[HCO_3^-]

). The anion gap is calculated as

135 - (112 + 14) = 9 \text{ mEq/L}

, which is normal. To determine if respiratory compensation is appropriate, Winter's formula must be applied:

Expected\ PaCO_2 = 1.5 \times 14 + 8 = 29 \pm 2 \text{ mm Hg}

. The patient's measured

PaCO_2

is

35 \text{ mm Hg}

. Since the measured

PaCO_2

is higher than the expected range of

27-31 \text{ mm Hg}

, the patient has a concurrent respiratory acidosis. This clinical picture is consistent with severe diarrhea causing bicarbonate loss (normal anion gap metabolic acidosis) alongside fentanyl-induced hypoventilation (respiratory acidosis).

Step-by-Step Solution

1

Analyze the pH and serum bicarbonate to identify the primary disorder.

The

pH

is

7.25

(acidemia) and

[HCO_3^-]

is

14 \text{ mEq/L}

(low). This indicates a primary metabolic acidosis.

Establishing the primary acid-base disturbance is the necessary first step before evaluating compensation.

2

Calculate the serum anion gap using the formula:

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

.

AG = 135 - (112 + 14) = 135 - 126 = 9 \text{ mEq/L}

. This is a normal anion gap (normal range is typically

8-12 \text{ mEq/L}

).

Differentiating between normal and high anion gap metabolic acidosis helps narrow down the etiology. Here, bicarbonate loss from severe diarrhea causes a normal anion gap metabolic acidosis (NAGMA).

3

Calculate the expected respiratory compensation using Winter's formula:

Expected\ PaCO_2 = (1.5 \times [HCO_3^-]) + 8 \pm 2

.

Expected\ PaCO_2 = (1.5 \times 14) + 8 = 21 + 8 = 29 \pm 2

. The expected range is

27

to

31 \text{ mm Hg}

.

Winter's formula determines if the respiratory response (hyperventilation blowing off CO2) is adequate for the degree of metabolic acidosis.

4

Compare the expected

PaCO_2

to the patient's measured

PaCO_2

.

The measured

PaCO_2

(

35 \text{ mm Hg}

) is greater than the expected maximum of

31 \text{ mm Hg}

.

Because the actual

PaCO_2

is higher than the expected compensatory range, the patient is retaining CO2. This indicates an independent, superimposed primary respiratory acidosis, likely secondary to fentanyl-induced respiratory depression (evidenced by a respiratory rate of 9/min).

Key Concept

In patients with metabolic acidosis, Winter's formula must be used to calculate expected PaCO2 to rule out a mixed acid-base disorder; an actual PaCO2 higher than expected signifies a concurrent respiratory acidosis.

Question 246Question →

An 81-year-old female resident of an assisted living facility is evaluated in the emergency department for severe, diffuse abdominal pain and intractable vomiting that began abruptly 5 hours ago. Her medical history includes hypertension and paroxysmal atrial fibrillation, for which she frequently misses her apixaban doses. On examination, the patient is agitated, diaphoretic, and writhing in pain. Abdominal examination reveals mild diffuse tenderness without guarding or rebound, which appears surprisingly benign given her reported level of pain.

Vitals:
* Temperature: $37.2^{\circ}\text{C} (99.0^{\circ}\text{F})$
* Pulse: $122/\text{min}$ , irregularly irregular
* Respirations: $32/\text{min}$
* Blood pressure: $148/85 \text{ mm Hg}$

Laboratory analysis reveals the following:

Serum Chemistry	Value
Sodium ( $\text{Na}^+$ )	$140 \text{ mEq/L}$
Potassium ( $\text{K}^+$ )	$3.2 \text{ mEq/L}$
Chloride ( $\text{Cl}^-$ )	$90 \text{ mEq/L}$
Bicarbonate ( $\text{HCO}_3^-$ )	$14 \text{ mEq/L}$
BUN	$28 \text{ mg/dL}$
Creatinine	$1.3 \text{ mg/dL}$

Arterial Blood Gas (Room Air)	Value
pH	$7.47$
$\text{pCO}_2$	$20 \text{ mm Hg}$
$\text{pO}_2$	$92 \text{ mm Hg}$

What is the most accurate interpretation of this patient's acid-base status?

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

High anion gap metabolic acidosis and respiratory alkalosis

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

Respiratory alkalosis and metabolic alkalosis

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

Show answer & explanation

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

Answer

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

The correct interpretation identifies a triple acid-base disorder. First, the anion gap is profoundly elevated at

36 \text{ mEq/L}

(

140 - (90 + 14)

), diagnosing a High Anion Gap Metabolic Acidosis (HAGMA), likely due to lactic acidosis from acute mesenteric ischemia. Second, applying Winter's formula (

1.5 \times 14 + 8 = 29

) shows the expected

\text{pCO}_2

should be

27-31 \text{ mm Hg}

. The actual

\text{pCO}_2

is

20 \text{ mm Hg}

, revealing an independent, concurrent Respiratory Alkalosis (driven by severe pain and hyperventilation). Finally, calculating the Delta Ratio (

\Delta\text{AG} / \Delta\text{HCO}_3^-

) yields

24 / 10 = 2.4

. A ratio

> 2.0

demonstrates that the bicarbonate level is inappropriately high for the degree of HAGMA, indicating a third, concurrent Metabolic Alkalosis (caused by intractable vomiting).

Step-by-Step Solution

1

Analyze the pH to determine the overall acid-base direction.

The pH is

7.47

, which indicates an overall alkalemia.

Establishing the net pH state is the first step, though significant hidden disorders can exist behind a misleading overall pH.

2

Calculate the serum Anion Gap (AG) to check for a hidden metabolic acidosis.

AG = \text{Na}^+ - (\text{Cl}^- + \text{HCO}_3^-) = 140 - (90 + 14) = 36

.

An

AG > 12 \text{ mEq/L}

confirms the presence of a primary High Anion Gap Metabolic Acidosis (HAGMA), regardless of the alkalemic pH.

3

Calculate the expected respiratory compensation using Winter's formula.

Expected

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

(range

27-31 \text{ mm Hg}

). Actual

\text{pCO}_2

is

20 \text{ mm Hg}

.

Since the actual

\text{pCO}_2

(

20

) is lower than the expected range (

27-31

), there is a concurrent primary Respiratory Alkalosis.

4

Calculate the Delta Ratio (

\Delta\text{AG} / \Delta\text{HCO}_3^-

) to uncover additional metabolic disorders.

\Delta\text{AG} = 36 - 12 = 24

.

\Delta\text{HCO}_3^- = 24 - 14 = 10

. Delta Ratio

= 24 / 10 = 2.4

.

A Delta Ratio

> 2.0

indicates that the bicarbonate has not dropped as much as expected given the massive influx of unmeasured anions, proving the presence of a concurrent Metabolic Alkalosis.

Key Concept

Diagnosis of mixed (triple) acid-base disorders using Anion Gap, Winter's formula, and Delta Ratio.

Estimated Time:2m 0s

Question 247Question →

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

Click a left item, then click its matching right item

Show answer & explanation

Answer

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.

Step-by-Step Solution

1

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.

2

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.

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.

4

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.

Key Concept

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

Question 248Question →

A 35-year-old woman presents for follow-up of generalized weakness and worsening fatigue over the last two weeks. Her medical history is notable for Sjögren syndrome, for which she takes hydroxychloroquine. She appears comfortable and her vital signs are stable. Muscle strength testing demonstrates 4/5 strength in the bilateral quadriceps and hip flexors. Given her symptoms, laboratory testing is obtained:

Serum:
Sodium: 140 mEq/L
Potassium: 2.8 mEq/L
Chloride: 115 mEq/L
Bicarbonate: 15 mEq/L
Blood urea nitrogen: 14 mg/dL
Creatinine: 0.8 mg/dL
Glucose: 95 mg/dL

Urine:
Sodium: 60 mEq/L
Potassium: 25 mEq/L
Chloride: 45 mEq/L
pH: 6.0

To determine the etiology of this patient's metabolic acidosis, the physician decides to calculate the urine anion gap. What is the value of this patient's urine anion gap?

Show answer & explanation

Answer: 40

Answer

40 mEq/L

The patient has a normal anion gap metabolic acidosis (NAGMA), as her serum anion gap is

140 - (115 + 15) = 10

mEq/L (normal: 8-12 mEq/L). In the setting of NAGMA, the urine anion gap (UAG) helps differentiate between renal loss of bicarbonate (e.g., renal tubular acidosis) and extrarenal loss (e.g., diarrhea). The formula for UAG is

U_{Na^+} + U_{K^+} - U_{Cl^-}

. Plugging in this patient's urinary values:

60 \text{ mEq/L} + 25 \text{ mEq/L} - 45 \text{ mEq/L} = 40 \text{ mEq/L}

. A positive UAG indicates impaired renal

NH_4^+

excretion, which is consistent with renal tubular acidosis (Type 1 RTA in this patient with Sjögren syndrome and severe hypokalemia).

Step-by-Step Solution

1

Identify the formula for urine anion gap.

UAG = U_{Na^+} + U_{K^+} - U_{Cl^-}

The urine anion gap is used to indirectly measure urinary ammonium (

NH_4^+

) excretion in a patient with a normal anion gap metabolic acidosis.

2

Extract the appropriate urinary electrolyte values from the laboratory data.

U_{Na^+} = 60

mEq/L,

U_{K^+} = 25

mEq/L,

U_{Cl^-} = 45

mEq/L

These are the measured urinary cations and anions needed for the calculation. Care must be taken not to mistakenly use serum values.

3

Calculate the urine anion gap.

60 + 25 - 45 = 40

mEq/L

Adding the cations and subtracting the chloride yields a positive value of 40 mEq/L, indicating impaired renal ammonium excretion (typical of distal renal tubular acidosis).

Key Concept

The urine anion gap (

U_{Na^+} + U_{K^+} - U_{Cl^-}

) is calculated to differentiate between gastrointestinal and renal causes of normal anion gap metabolic acidosis; a positive value suggests a renal etiology such as renal tubular acidosis.

Question 249Question →

A 45-year-old woman comes to the clinic reporting progressive muscle weakness and recurrent cramping in her legs for the past three weeks. She also notes a several-month history of a sensation of "sand in her eyes" and a dry mouth requiring frequent sips of water. She takes no medications. Vital signs are within normal limits. Physical examination shows 4/5 motor strength in the lower extremities bilaterally with normal deep tendon reflexes.

Laboratory results are as follows:

Test	Result
Serum chemistry
Sodium	140 mEq/L
Potassium	2.2 mEq/L
Chloride	115 mEq/L
Bicarbonate	13 mEq/L
Blood urea nitrogen	14 mg/dL
Creatinine	0.8 mg/dL
Arterial blood gas (room air)
pH	7.25
PaCO2	27 mmHg
PaO2	95 mmHg
Urine studies
pH	6.5
Sodium	30 mEq/L
Potassium	15 mEq/L
Chloride	20 mEq/L

Which of the following is the primary mechanism responsible for this patient's acid-base disturbance?

Impaired hydrogen ion secretion in the alpha-intercalated cells

Decreased bicarbonate reabsorption in the proximal convoluted tubule

Aldosterone resistance in the cortical collecting duct

Excessive gastrointestinal loss of bicarbonate

Increased production of unmeasured organic acids

Show answer & explanation

Answer: Impaired hydrogen ion secretion in the alpha-intercalated cells

Answer

Impaired hydrogen ion secretion in the alpha-intercalated cells

The patient presents with a normal anion gap metabolic acidosis (NAGMA), evidenced by a calculated serum anion gap of

12

mEq/L (

140 - [115 + 13]

). In the context of NAGMA, a positive urine anion gap (

UAG = 30 + 15 - 20 = +25

mEq/L) indicates a failure of the kidneys to properly excrete ammonium, which distinguishes a Renal Tubular Acidosis (RTA) from gastrointestinal bicarbonate losses (e.g., diarrhea). The presence of severe hypokalemia and an inappropriately alkaline urine (

pH = 6.5

) during systemic acidemia isolates the defect specifically to Type 1 (distal) RTA. Type 1 RTA is caused by impaired secretion of hydrogen ions by the alpha-intercalated cells of the cortical collecting duct. Furthermore, the patient's sicca symptoms (dry eyes, dry mouth) are highly suggestive of underlying Sjögren syndrome, a classic autoimmune condition strongly associated with Type 1 RTA.

Step-by-Step Solution

1

Analyze the basic metabolic panel and arterial blood gas to identify the primary acid-base disorder.

The pH of 7.25 and bicarbonate of 13 mEq/L confirm metabolic acidosis. The calculated serum anion gap is 12 mEq/L (

140 - [115 + 13]

), indicating a normal anion gap metabolic acidosis (NAGMA). Expected

PaCO_2

by Winter's formula is

27.5 \pm 2

mmHg, matching the measured

PaCO_2

of 27 mmHg and indicating appropriate respiratory compensation.

This establishes the foundational diagnosis and rules out superimposed respiratory disorders or unmeasured organic acids.

2

Calculate the urine anion gap (UAG) to determine the etiology of the NAGMA.

The UAG is

+25

mEq/L (

30 + 15 - 20

).

A positive UAG in the setting of NAGMA indicates impaired renal ammonium excretion, differentiating a renal etiology (Renal Tubular Acidosis) from gastrointestinal bicarbonate loss (which would yield a negative UAG).

3

Differentiate the specific type of Renal Tubular Acidosis (RTA) based on potassium and urine pH.

The patient has severe hypokalemia (

2.2

mEq/L), ruling out Type 4 RTA. The urine pH is inappropriately high (

6.5

) despite severe systemic acidemia.

In Type 1 RTA, the distal tubule completely fails to secrete hydrogen ions, leading to an inability to maximally acidify the urine (pH

> 5.5

). In Type 2 RTA, the distal tubule remains intact and can usually acidify the urine when systemic acidosis is this severe.

4

Correlate the pathophysiologic findings with the patient's clinical presentation.

The patient's chronic dry eyes and dry mouth strongly suggest Sjögren syndrome.

Sjögren syndrome is classically associated with Type 1 RTA due to autoantibody-mediated damage to the distal tubular cells, confirming the mechanism.

Key Concept

Type 1 (distal) Renal Tubular Acidosis is characterized by a normal anion gap metabolic acidosis, severe hypokalemia, a positive urine anion gap, and an inability to acidify urine (urine pH > 5.5). It is caused by a defect in hydrogen ion secretion by the alpha-intercalated cells and is frequently associated with autoimmune diseases such as Sjögren syndrome.

Question 250Question →

A 56-year-old man with a history of alcohol use disorder and decompensated cirrhosis is admitted to the hospital with worsening lethargy and diffuse abdominal swelling over the past week. His temperature is $37.2^\circ\text{C}$ ( $99.0^\circ\text{F}$ ), blood pressure is $102/64 \text{ mm Hg}$ , pulse is $108/\text{min}$ , and respiratory rate is $22/\text{min}$ . Physical examination reveals scleral icterus, shifting dullness to percussion, and bilateral pitting edema. Laboratory studies are obtained:

Laboratory Parameter	Value
Sodium ( $Na^+$ )	$138 \text{ mEq/L}$
Potassium ( $K^+$ )	$3.8 \text{ mEq/L}$
Chloride ( $Cl^-$ )	$107 \text{ mEq/L}$
Bicarbonate ( $HCO_3^-$ )	$17 \text{ mEq/L}$
Blood urea nitrogen	$32 \text{ mg/dL}$
Creatinine	$1.5 \text{ mg/dL}$
Albumin	$1.6 \text{ g/dL}$

Assuming a normal reference serum albumin of $4.0 \text{ g/dL}$ , what is this patient's albumin-corrected serum anion gap?

Show answer & explanation

Answer: 20

Answer

20

The correct calculation yields

20 \text{ mEq/L}

. The patient's observed anion gap is

14 \text{ mEq/L}

(

138 - [107 + 17]

). Because the patient has severe hypoalbuminemia (

1.6 \text{ g/dL}

), this observed value is misleadingly low. To find the true anion gap, a correction of

2.5 \text{ mEq/L}

is added for every

1.0 \text{ g/dL}

that the albumin is below the reference of

4.0 \text{ g/dL}

. The albumin deficit is

2.4 \text{ g/dL}

(

4.0 - 1.6

). Multiplying

2.4

by

2.5

gives a correction factor of

6.0 \text{ mEq/L}

. Adding this to the observed gap of

14 \text{ mEq/L}

gives a true, corrected anion gap of

20 \text{ mEq/L}

, revealing a hidden high anion gap metabolic acidosis (likely due to lactic acidosis, given his elevated heart rate, low blood pressure, and severe liver disease).

Step-by-Step Solution

1

Calculate the uncorrected serum anion gap.

138 - (107 + 17) = 138 - 124 = 14 \text{ mEq/L}

The standard anion gap estimates unmeasured anions in the blood, primarily driven by the difference between major measured cations and anions.

2

Identify the patient's albumin deficit.

4.0 \text{ g/dL} - 1.6 \text{ g/dL} = 2.4 \text{ g/dL}

Albumin is a negatively charged protein and represents a major portion of the baseline unmeasured anions. Hypoalbuminemia artificially lowers the baseline anion gap.

3

Calculate the albumin correction factor.

2.4 \times 2.5 = 6.0 \text{ mEq/L}

For every

1.0 \text{ g/dL}

drop in serum albumin below the normal

4.0 \text{ g/dL}

, the anion gap is falsely lowered by approximately

2.5 \text{ mEq/L}

.

4

Add the correction factor to the observed anion gap.

14 + 6.0 = 20 \text{ mEq/L}

Adding the lost unmeasured anion equivalent back to the observed gap reveals the true metabolic state, uncovering a high anion gap metabolic acidosis.

Key Concept

In patients with significant hypoalbuminemia (such as those with cirrhosis or nephrotic syndrome), the serum anion gap must be corrected upward (

2.5 \text{ mEq/L}

for every

1 \text{ g/dL}

decrease in albumin below

4.0 \text{ g/dL}

) to accurately identify an underlying high anion gap metabolic acidosis.

Question 251Question →

Evaluate the provided arterial blood gas (ABG) and basic metabolic panel (BMP) profiles. Which clinical etiology most accurately corresponds to each laboratory pattern?

Click a left item, then click its matching right item

Show answer & explanation

Answer

Match each laboratory profile to its physiological driver by sequentially identifying the primary disturbance (via pH and

PaCO_2

/

HCO_3^-

direction) and calculating the anion gap for any metabolic acidosis.

By systematically interpreting the pH,

PaCO_2

, and

HCO_3^-

, along with calculating the anion gap for acidemic patients, the specific primary acid-base disturbance can be definitively identified and matched to its classic clinical trigger.

Step-by-Step Solution

1

Determine the primary acid-base disturbance for each profile using the pH.

Profiles 1, 3, and 4 exhibit acidemia (pH

< 7.35

). Profiles 2 and 5 exhibit alkalemia (pH

> 7.45

).

The net pH deviation dictates whether the primary underlying process is an acidosis or an alkalosis.

2

Differentiate respiratory versus metabolic drivers, and calculate the anion gap for metabolic acidoses.

Profile 1: Respiratory Acidosis. Profile 2: Metabolic Alkalosis. Profile 3: Normal Anion Gap Metabolic Acidosis (AG

= 12

). Profile 4: High Anion Gap Metabolic Acidosis (AG

= 26

). Profile 5: Respiratory Alkalosis.

Comparing

PaCO_2

and

HCO_3^-

shifts against the pH identifies the driver. The anion gap formula (

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

) is mandatory to categorize metabolic acidosis.

3

Correlate the derived primary disorder with the appropriate clinical etiology.

Hypoventilation (opiate) correlates to Respiratory Acidosis. Vomiting correlates to Metabolic Alkalosis. Fistula output correlates to NAGMA. Ketoacidosis correlates to HAGMA. Hyperventilation (panic) correlates to Respiratory Alkalosis.

Specific clinical scenarios generate predictable, corresponding shifts in serum electrolytes and arterial blood gas values.

Key Concept

Identification and etiology of simple acid-base disorders based on laboratory parameters

Estimated Time:2m 0s

Question 252Question →

A 24-year-old woman is brought to the emergency department due to severe nausea, diffuse abdominal pain, and confusion. She has a history of type 1 diabetes mellitus but ran out of her insulin prescription three days ago. For the past 48 hours, she has had intractable vomiting, rendering her unable to tolerate oral intake. Her temperature is 37.1°C (98.8°F), blood pressure is 98/60 mm Hg, pulse is 118/min, and respirations are 24/min and deep. Physical examination reveals dry mucous membranes, decreased skin turgor, and diffuse abdominal tenderness without rebound or guarding.

Laboratory studies reveal:

Serum chemistry	Value
Sodium	140 mEq/L
Potassium	3.2 mEq/L
Chloride	94 mEq/L
Bicarbonate	16 mEq/L
Blood urea nitrogen	32 mg/dL
Creatinine	1.4 mg/dL
Glucose	480 mg/dL

Arterial blood gas (room air):

Parameter	Value
pH	7.33
pCO2	32 mm Hg
pO2	92 mm Hg

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

Anion gap metabolic acidosis and concurrent metabolic alkalosis

Isolated anion gap metabolic acidosis with appropriate respiratory compensation

Anion gap metabolic acidosis and concurrent respiratory acidosis

Anion gap metabolic acidosis and concurrent respiratory alkalosis

Non-anion gap metabolic acidosis and concurrent metabolic alkalosis

Show answer & explanation

Answer: Anion gap metabolic acidosis and concurrent metabolic alkalosis

Answer

Anion gap metabolic acidosis and concurrent metabolic alkalosis

This patient has a mixed acid-base disorder comprising an anion gap metabolic acidosis (AGMA) and a metabolic alkalosis, with appropriate respiratory compensation. First, the pH of 7.33 and low bicarbonate (16 mEq/L) indicate a primary metabolic acidosis. Next, calculating the anion gap (

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

) yields

140 - (94 + 16) = 30

mEq/L, indicating a severe AGMA, consistent with diabetic ketoacidosis. To determine if a secondary metabolic disorder is present, the delta gap must be analyzed. The anion gap increased by 18 (

30 - 12

), so the bicarbonate should drop by exactly 18 to 6 mEq/L (

24 - 18 = 6

). However, the patient's actual bicarbonate is 16 mEq/L. Because the actual bicarbonate is significantly higher than the predicted bicarbonate, there is a concurrent metabolic alkalosis, likely secondary to her intractable vomiting (loss of gastric HCl). Finally, Winter's formula (

\text{pCO}_2 = 1.5 \times \text{HCO}_3^- + 8 \pm 2

) yields an expected

\text{pCO}_2

of

1.5 \times 16 + 8 = 32

mm Hg. Because the measured

\text{pCO}_2

is exactly 32 mm Hg, respiratory compensation is appropriate and there is no concurrent respiratory disorder.

Step-by-Step Solution

1

Determine the primary disturbance from the pH and bicarbonate.

The patient has acidemia (pH 7.33) with a low bicarbonate (16 mEq/L), indicating a primary metabolic acidosis.

Identifying the primary disorder provides the baseline for further acid-base calculations.

2

Calculate the anion gap (AG).

\text{AG} = \text{Na}^+ - (\text{Cl}^- + \text{HCO}_3^-) = 140 - (94 + 16) = 30

mEq/L.

A significantly elevated AG (> 12 mEq/L) confirms an anion gap metabolic acidosis, typical of diabetic ketoacidosis.

3

Check respiratory compensation using Winter's formula.

Expected

\text{pCO}_2 = 1.5 \times 16 + 8 = 32

mm Hg. The actual

\text{pCO}_2

is 32 mm Hg.

Because the actual

\text{pCO}_2

falls exactly within the expected range, there is appropriate respiratory compensation without a concurrent primary respiratory disorder.

4

Calculate the delta gap to identify hidden metabolic disorders.

\Delta\text{AG} = 30 - 12 = 18

. Expected

\text{HCO}_3^- = 24 - 18 = 6

mEq/L. Actual

\text{HCO}_3^-

is 16 mEq/L.

Because the actual bicarbonate (16) is greater than the expected bicarbonate (6), a concurrent metabolic alkalosis is present, consistent with her history of intractable vomiting.

Key Concept

In complex patients (e.g., DKA with vomiting), evaluating the delta gap (or delta ratio) is required after calculating the anion gap to unmask hidden concurrent metabolic disorders.

Alternative Method

Instead of subtracting the delta gap from a normal bicarbonate, you can calculate the delta ratio (

\frac{\Delta\text{AG}}{\Delta\text{HCO}_3^-}

). A ratio between 1.0 and 2.0 indicates an isolated anion gap metabolic acidosis, while a ratio > 2.0 indicates a concurrent metabolic alkalosis. Here, the ratio is

\frac{18}{8} = 2.25

, confirming the concurrent metabolic alkalosis.

Estimated Time:2m 0s

Question 253Question →

A $20$ -year-old woman is brought to the emergency department by her roommate due to confusion, nausea, and ringing in her ears that began several hours ago. The roommate found an empty bottle of an over-the-counter analgesic in their shared bathroom. The patient has a medical history of major depressive disorder. Her temperature is $38.0^\circ$ C ( $100.4^\circ$ F), blood pressure is $112/70$ mm Hg, pulse is $114$ /min, and respirations are $28$ /min. She appears agitated and is breathing deeply and rapidly. Arterial blood gas on room air and serum chemistry results are as follows:

Laboratory Parameter	Result
$pH$	$7.44$
$PaCO_2$	$16$ mm Hg
$PaO_2$	$98$ mm Hg
Sodium ( $Na^+$ )	$140$ mEq/L
Potassium ( $K^+$ )	$3.6$ mEq/L
Chloride ( $Cl^-$ )	$104$ mEq/L
Bicarbonate ( $HCO_3^-$ )	$10$ mEq/L
Blood urea nitrogen	$16$ mg/dL
Creatinine	$0.8$ mg/dL

Which of the following best describes 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 normal anion gap metabolic acidosis and primary respiratory alkalosis

Primary high anion gap metabolic acidosis and concurrent primary respiratory acidosis

Show answer & explanation

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

Answer

Mixed primary high anion gap metabolic acidosis and primary respiratory alkalosis

The correct answer accurately identifies two independent primary acid-base disorders using systematic calculation. First, the anion gap is

26

mEq/L (

140 - [104 + 10]

), which defines a high anion gap metabolic acidosis. Second, applying Winter's formula (

1.5 \times 10 + 8 = 23 \pm 2

mm Hg) reveals that adequate respiratory compensation should result in a

PaCO_2

between

21

and

25

mm Hg. Because the patient's actual

PaCO_2

is

16

mm Hg, she is 'blowing off' more

CO_2

than expected, proving a concurrent primary respiratory alkalosis. This mixed picture, along with tinnitus and hyperthermia, is the hallmark of salicylate toxicity.

Step-by-Step Solution

1

Calculate the Anion Gap to determine the presence and type of metabolic acidosis.

AG = Na^+ - (Cl^- + HCO_3^-) = 140 - (104 + 10) = 26

mEq/L.

An elevated anion gap (

>12

mEq/L) establishes the presence of a primary high anion gap metabolic acidosis (HAGMA).

2

Calculate the expected respiratory compensation for the metabolic acidosis using Winter's formula.

Expected

PaCO_2 = 1.5 \times HCO_3^- + 8 \pm 2 = (1.5 \times 10) + 8 = 23 \pm 2

mm Hg (Range:

21-25

mm Hg).

This determines if the change in

PaCO_2

is purely compensatory or if there is a secondary respiratory acid-base disorder.

3

Compare the actual

PaCO_2

to the expected

PaCO_2

calculated from Winter's formula.

Actual

PaCO_2

(

16

mm Hg) is less than the expected range (

21-25

mm Hg).

Because the patient is hyperventilating more than expected for her level of metabolic acidosis, there is a concurrent primary respiratory alkalosis.

4

Calculate the Delta Ratio to check for a third hidden disorder.

\Delta AG = 26 - 12 = 14

.

\Delta HCO_3^- = 24 - 10 = 14

. Ratio =

14/14 = 1.0

.

A ratio of

1.0

confirms a pure high anion gap metabolic acidosis with no hidden normal anion gap acidosis or metabolic alkalosis.

Key Concept

Salicylate toxicity classically presents with a mixed acid-base disorder: a primary respiratory alkalosis (due to direct stimulation of the medullary respiratory center) and a primary high anion gap metabolic acidosis (due to uncoupling of oxidative phosphorylation and accumulation of organic acids).

Estimated Time:2m 0s

Question 254Question →

A 54-year-old man is brought to the emergency department by emergency medical services after his wife found him unresponsive in their garage. She reports that he has a history of major depressive disorder and alcohol use disorder. An open, half-empty bottle of engine antifreeze was found nearby. Vital signs show a temperature of $36.2^\circ\text{C}$ ( $97.2^\circ\text{F}$ ), pulse of $105$ /min, blood pressure of $145/88$ mm Hg, and respirations of $26$ /min. Physical examination reveals a comatose patient with no focal neurological deficits.

Laboratory studies reveal the following:

Parameter	Value
Sodium	$141$ mEq/L
Potassium	$4.2$ mEq/L
Chloride	$104$ mEq/L
Bicarbonate	$8$ mEq/L
Blood urea nitrogen	$42$ mg/dL
Creatinine	$1.8$ mg/dL
Glucose	$126$ mg/dL
Ethanol	Undetectable
Measured serum osmolality	$340$ mOsm/kg $\text{H}_2\text{O}$

Arterial blood gas on room air:
pH: $7.18$
$\text{pCO}_2$ : $20$ mm Hg
$\text{pO}_2$ : $98$ mm Hg

Urinalysis reveals the presence of envelope-shaped crystals.

Based on the provided laboratory values, what is the patient's serum osmolar gap in mOsm/kg $\text{H}_2\text{O}$ ?

Show answer & explanation

Answer: 36

Answer

36

The patient's osmolar gap is

36

mOsm/kg

\text{H}_2\text{O}

. This is obtained by first finding the calculated serum osmolality using the formula:

2[\text{Na}^+] + \frac{\text{Glucose}}{18} + \frac{\text{BUN}}{2.8}

. Substituting the provided values yields

2(141) + \frac{126}{18} + \frac{42}{2.8} = 282 + 7 + 15 = 304

mOsm/kg

\text{H}_2\text{O}

. Subtracting this calculated value from the measured osmolality (

340

mOsm/kg

\text{H}_2\text{O}

) gives a gap of

36

mOsm/kg

\text{H}_2\text{O}

.

Step-by-Step Solution

1

Determine the formula for calculated serum osmolality.

\text{Calculated Osmolality} = 2[\text{Na}^+] + \frac{\text{Glucose}}{18} + \frac{\text{BUN}}{2.8}

.

This standard calculation is required when assessing a patient for suspected toxic alcohol ingestion.

2

Input the patient's laboratory values into the formula.

\text{Calculated Osmolality} = 2(141) + \frac{126}{18} + \frac{42}{2.8} = 282 + 7 + 15 = 304

mOsm/kg

\text{H}_2\text{O}

.

Dividing glucose by 18 and BUN by 2.8 correctly converts these values from mg/dL to mOsm/L so they can be summed with sodium.

3

Calculate the serum osmolar gap.

\text{Osmolar Gap} = \text{Measured Osmolality} - \text{Calculated Osmolality} = 340 - 304 = 36

mOsm/kg

\text{H}_2\text{O}

.

The difference between measured and calculated osmolality reveals the concentration of unmeasured osmotically active substances (e.g., ethylene glycol).

Key Concept

The serum osmolar gap is calculated by subtracting the calculated serum osmolality (

2[\text{Na}^+] + \frac{\text{Glucose}}{18} + \frac{\text{BUN}}{2.8}

) from the measured serum osmolality. A gap

>10

mOsm/kg

\text{H}_2\text{O}

suggests the presence of unmeasured osmotically active agents such as toxic alcohols (e.g., ethylene glycol, methanol).

Question 255Question →

A 24-year-old woman presents to the emergency department with a 2-day history of severe right-sided flank pain, nausea, chills, and fever. She has no significant past medical history and takes no medications. On presentation, her temperature is 39.4°C (102.9°F), blood pressure is 98/55 mm Hg, pulse is 128/min, and respirations are 32/min. Physical examination reveals exquisite right costovertebral angle tenderness and a flushed appearance.

Laboratory results are as follows:

Parameter	Value
Sodium	138 mEq/L
Potassium	4.2 mEq/L
Chloride	102 mEq/L
Bicarbonate	14 mEq/L
Blood urea nitrogen	22 mg/dL
Serum creatinine	1.3 mg/dL
Serum lactate	4.2 mmol/L

Arterial blood gas analysis on room air reveals:

Parameter	Value
pH	7.32
$pCO_2$	22 mm Hg
$pO_2$	96 mm Hg

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

High anion gap metabolic acidosis and concomitant respiratory alkalosis

High anion gap metabolic acidosis with appropriate respiratory compensation

Normal anion gap metabolic acidosis and concomitant respiratory alkalosis

High anion gap metabolic acidosis and concomitant respiratory acidosis

Primary respiratory alkalosis with appropriate metabolic compensation

Show answer & explanation

Answer: High anion gap metabolic acidosis and concomitant respiratory alkalosis

Answer

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

The correct characterization is a high anion gap metabolic acidosis (HAGMA) with a concomitant respiratory alkalosis. The patient is acidemic (pH < 7.40) with a low bicarbonate, indicating primary metabolic acidosis. The anion gap is elevated at 22 mEq/L (due to lactic acid production from early sepsis/pyelonephritis). Using Winter's formula, the expected

pCO_2

is

(1.5 \times 14) + 8 = 29 \pm 2

(range: 27-31 mm Hg). The measured

pCO_2

is 22 mm Hg. Because the actual

pCO_2

is significantly lower than the expected compensatory range, an independent primary respiratory alkalosis is also present, driven by central stimulation of respiration from systemic inflammatory response syndrome (SIRS), fever, and pain.

Step-by-Step Solution

1

Determine the primary acid-base disorder based on the pH.

The pH is 7.32, which is less than 7.40, indicating acidemia. The low bicarbonate (14 mEq/L) aligns with the low pH, confirming a primary metabolic acidosis.

Establishing the primary disturbance directs which compensation formula must be applied.

2

Calculate the serum anion gap.

AG = Na^+ - (Cl^- + HCO_3^-) = 138 - (102 + 14) = 22

. Normal is roughly 10-14 mEq/L, indicating a High Anion Gap Metabolic Acidosis (HAGMA).

Differentiating between normal and high anion gap narrows down the underlying etiology (in this case, lactic acidosis due to sepsis).

3

Apply Winter's formula to calculate the expected respiratory compensation.

Expected

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

.
Expected

pCO_2 = (1.5 \times 14) + 8 = 21 + 8 = 29 \pm 2

.
The expected

pCO_2

range is

27

to

31

mm Hg.

This establishes the target

pCO_2

baseline if the lungs were purely compensating for the metabolic derangement.

4

Compare the actual

pCO_2

to the expected

pCO_2

.

The patient's actual

pCO_2

is 22 mm Hg. Since

22 < 27

, the patient is hyperventilating more than expected.

A

pCO_2

lower than the expected compensatory range indicates a separate, primary respiratory alkalosis (often driven by pain, fever, or early sepsis stimulating the respiratory center).

Key Concept

In patients with metabolic acidosis, Winter's formula (

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

) must be used to calculate expected respiratory compensation. An actual

pCO_2

lower than the expected range indicates a concomitant primary respiratory alkalosis.

Alternative Method

To ensure absolute precision, you can also calculate the Delta Ratio (

\Delta AG / \Delta HCO_3^-

).

\Delta AG = 22 - 12 = 10

.

\Delta HCO_3^- = 24 - 14 = 10

. The ratio is 1.0. A ratio between 1.0 and 2.0 confirms a pure high anion gap metabolic acidosis without a hidden normal anion gap acidosis or metabolic alkalosis.

Estimated Time:1m 30s

Question 256Question →

A 68-year-old man is evaluated in the intensive care unit for oliguria. He was admitted 4 days ago for severe acute pancreatitis complicated by shock, which required aggressive intravenous fluid resuscitation and transient vasopressor support. His blood pressure is now 112/70 mm Hg and his heart rate is 88/min on no vasoactive medications. Urine output over the past 24 hours has been 280 mL. Laboratory studies show the following:

Serum values:
- Sodium: $140$ mEq/L
- Potassium: $4.5$ mEq/L
- Chloride: $105$ mEq/L
- Bicarbonate: $21$ mEq/L
- Blood urea nitrogen: $48$ mg/dL
- Creatinine: $2.8$ mg/dL (baseline $0.9$ mg/dL)

Urine values:
- Sodium: $50$ mEq/L
- Creatinine: $50$ mg/dL

Urinalysis demonstrates the presence of muddy brown granular casts. Based on these findings, what is this patient's calculated fractional excretion of sodium (FENa) in percentage?

Show answer & explanation

Answer: 2

Answer

The patient's fractional excretion of sodium (FENa) is 2%.

The correct calculation yields a fractional excretion of sodium of 2%. This patient has developed acute tubular necrosis (ATN), a form of intrinsic renal failure, as a complication of prolonged hypotension/shock (ischemic ATN). Intrinsic renal failure is characterized by structural damage to the renal tubules (evidenced by muddy brown granular casts). Because the damaged tubules cannot adequately reabsorb sodium, the FENa is typically > 2% in ATN. FENa is determined by the formula:

FENa = \frac{U_{Na} \times P_{Cr}}{P_{Na} \times U_{Cr}} \times 100

. Substituting the values from the vignette gives

\frac{50 \times 2.8}{140 \times 50} \times 100 = 2\%

.

Step-by-Step Solution

1

Extract the four relevant laboratory values required for the FENa calculation.

U_{Na} = 50

,

P_{Cr} = 2.8

,

P_{Na} = 140

, and

U_{Cr} = 50

.

The fractional excretion of sodium relies exclusively on urine and serum concentrations of sodium and creatinine.

2

Apply the standard formula for FENa.

FENa = \frac{U_{Na} \times P_{Cr}}{P_{Na} \times U_{Cr}} \times 100

.

This formula effectively calculates the clearance of sodium divided by the clearance of creatinine, estimating the proportion of filtered sodium that is excreted.

3

Calculate the numerator.

50 \times 2.8 = 140

.

Multiplies urine sodium by serum creatinine.

4

Calculate the denominator.

140 \times 50 = 7000

.

Multiplies serum sodium by urine creatinine.

5

Divide the numerator by the denominator and multiply by 100 to get the percentage.

\frac{140}{7000} \times 100 = 0.02 \times 100 = 2\%

.

Yields the final percentage indicating the diagnostic category of the acute kidney injury.

Key Concept

Calculation and interpretation of Fractional Excretion of Sodium (FENa) to distinguish intrinsic acute tubular necrosis (ATN) from prerenal azotemia.

Question 257Question →

A 34-year-old female presents to the nephrology clinic for evaluation of profound, generalized muscle weakness that has progressively worsened over the past month. Her past medical history is significant for systemic lupus erythematosus (SLE) and two episodes of nephrolithiasis within the last year. She currently takes hydroxychloroquine. Vital signs are within normal limits. Physical examination is notable for symmetric proximal muscle weakness, with reduced deep tendon reflexes in the upper and lower extremities. No focal neurologic deficits are observed.

Laboratory studies are obtained:

Serum Chemistry	Result
Sodium	$140$ mEq/L
Potassium	$2.6$ mEq/L
Chloride	$115$ mEq/L
Bicarbonate	$15$ mEq/L
Blood urea nitrogen	$14$ mg/dL
Creatinine	$0.9$ mg/dL

Urine Studies	Result
pH	$6.6$
Specific gravity	$1.012$
Sodium	$50$ mEq/L
Potassium	$20$ mEq/L
Chloride	$30$ mEq/L
Microscopy	Frequent calcium phosphate crystals

Which of the following best describes the primary pathophysiology underlying this patient's laboratory abnormalities?

Impaired distal tubular secretion of hydrogen ions

Defective proximal tubular reabsorption of filtered bicarbonate

Excessive gastrointestinal loss of bicarbonate-rich fluids

Resistance to aldosterone at the cortical collecting duct

Inappropriate renal retention of unmeasured organic anions

Show answer & explanation

Answer: Impaired distal tubular secretion of hydrogen ions

Answer

Impaired distal tubular secretion of hydrogen ions

This patient has a normal anion gap metabolic acidosis (NAGMA), as her calculated serum anion gap is 10 mEq/L. The evaluation of a NAGMA requires calculating the urine anion gap (UAG = Na + K - Cl) to estimate urinary ammonium excretion. Her UAG is highly positive (+40), indicating impaired renal ammonium excretion. This points definitively to a Renal Tubular Acidosis (RTA) rather than gastrointestinal bicarbonate loss. The combination of hypokalemia, an inappropriately alkaline urine (pH > 5.5), and calcium phosphate nephrolithiasis in a patient with an autoimmune disease (SLE) is highly specific for Type 1 (distal) RTA. The underlying mechanism of Type 1 RTA is the impaired secretion of hydrogen ions by the alpha-intercalated cells of the distal tubule.

Step-by-Step Solution

1

Calculate the serum anion gap.

Anion Gap = Sodium - (Chloride + Bicarbonate) = 140 - (115 + 15) = 10 mEq/L (Normal).

This establishes that the patient has a normal anion gap metabolic acidosis (NAGMA).

2

Calculate the urine anion gap (UAG).

UAG = Urine Sodium + Urine Potassium - Urine Chloride = 50 + 20 - 30 = +40 mEq/L.

A positive UAG in the setting of NAGMA indicates impaired renal ammonium excretion, differentiating Renal Tubular Acidosis (RTA) from gastrointestinal bicarbonate loss (which would yield a negative UAG).

3

Differentiate the type of Renal Tubular Acidosis based on potassium levels and urine pH.

The patient has severe hypokalemia (rules out Type 4 RTA) and an inappropriately alkaline urine pH of 6.6 (rules out Type 2 RTA during acidemia).

This unique combination isolates the specific tubular defect to the distal nephron.

4

Correlate clinical findings with the identified RTA.

The findings represent Type 1 (distal) RTA, which causes calcium phosphate nephrolithiasis and is strongly associated with autoimmune diseases like SLE.

Identifying the disease allows for mapping to its core mechanism: impaired H+ secretion by alpha-intercalated cells.

Key Concept

Evaluation of normal anion gap metabolic acidosis and identification of Type 1 (distal) Renal Tubular Acidosis

Estimated Time:2m 0s

Question 258Question →

A nephrologist is evaluating laboratory data and urinalysis findings from several patients. Match each clinical laboratory presentation with its most likely underlying etiology.

Click a left item, then click its matching right item

Show answer & explanation

Answer

The presentations map to Prerenal azotemia (concentrated urine, low FENa), Acute tubular necrosis (muddy brown casts, high FENa), Type 1 RTA (high urine pH, kidney stones), Type 4 RTA (hyperkalemia, diabetes), and Acute glomerulonephritis (RBC casts), respectively.

Each laboratory and clinical profile correctly corresponds to a specific nephrologic disorder based on pathophysiologic mechanisms of sodium handling, concentrating ability, acid-base balance, and glomerular integrity.

Step-by-Step Solution

1

Analyze the urinalysis and electrolyte parameters for the first two items to differentiate forms of acute kidney injury.

Low FENa and high osmolality indicate intact tubular function responding to hypoperfusion (prerenal). High FENa, low osmolality, and muddy brown casts indicate intrinsic tubular damage (acute tubular necrosis).

Differentiating prerenal from intrinsic renal failure relies on markers of tubular concentrating ability and sodium handling.

2

Evaluate the acid-base and electrolyte profiles for the next two items to distinguish forms of renal tubular acidosis (RTA).

A positive urine anion gap with high urine pH and stones points to Type 1 RTA. Hyperkalemia in a diabetic patient points to Type 4 RTA.

RTAs share non-anion gap metabolic acidosis but are differentiated by serum potassium, urine pH, and specific clinical associations.

3

Assess the urinary sediment in the final item.

RBC casts and dysmorphic RBCs indicate glomerular origin of hematuria.

RBC casts are pathognomonic for glomerulonephritis.

Key Concept

Interpretation of urinalysis, electrolyte panels, and acid-base status to diagnose renal etiologies.

Question 259Question →

A 68-year-old man is brought to the emergency department for worsening shortness of breath, increased sputum production, and confusion over the past 24 hours. He has very severe chronic obstructive pulmonary disease (COPD) and uses home oxygen therapy. The patient's wife reports that for the past 3 days, he has also had multiple episodes of large-volume, watery diarrhea.

Vital signs are temperature 37.6°C (99.7°F), blood pressure 110/70 mm Hg, pulse 115/min, and respirations 26/min. Physical examination shows a barrel chest, diffuse bilateral wheezing, and a prolonged expiratory phase. The mucous membranes are dry.

Laboratory results from today are as follows:

Serum chemistries:

Test	Result
Sodium	135 mEq/L
Potassium	3.5 mEq/L
Chloride	102 mEq/L
Bicarbonate	23 mEq/L
Blood urea nitrogen	28 mg/dL
Creatinine	1.2 mg/dL

Arterial blood gas:

Test	Result
pH	7.17
PaCO2	66 mm Hg
PaO2	50 mm Hg
HCO3-	23 mEq/L

A review of the patient's electronic medical record reveals an arterial blood gas obtained during a routine clinic visit 2 months ago when he was feeling well:

Baseline Arterial blood gas:

Test	Result
pH	7.36
PaCO2	56 mm Hg
PaO2	62 mm Hg
HCO3-	31 mEq/L

Which of the following combinations of acid-base disorders best describes this patient's current condition?

Acute-on-chronic respiratory acidosis and normal anion gap metabolic acidosis

Acute-on-chronic respiratory acidosis and high anion gap metabolic acidosis

Acute respiratory acidosis and normal anion gap metabolic acidosis

Chronic respiratory acidosis and normal anion gap metabolic acidosis

Uncompensated acute respiratory acidosis

Show answer & explanation

Answer: Acute-on-chronic respiratory acidosis and normal anion gap metabolic acidosis

Answer

Acute-on-chronic respiratory acidosis and normal anion gap metabolic acidosis

The correct answer is acute-on-chronic respiratory acidosis and normal anion gap metabolic acidosis. The patient's baseline ABG shows a compensated chronic respiratory acidosis (PaCO2

56 \text{ mm Hg}

, HCO3-

31 \text{ mEq/L}

). Today, his PaCO2 has acutely increased to

66 \text{ mm Hg}

(an acute increase of

10 \text{ mm Hg}

). For an acute rise in PaCO2, the HCO3- should increase by

1 \text{ mEq/L}

per

10 \text{ mm Hg}

rise in PaCO2. Therefore, his expected HCO3- is

32 \text{ mEq/L}

(

31 + 1

). His actual HCO3- is

23 \text{ mEq/L}

, which is significantly lower than expected, indicating a concurrent metabolic acidosis. The anion gap is calculated as

135 - (102 + 23) = 10 \text{ mEq/L}

, which is normal. This normal anion gap metabolic acidosis is clinically supported by his 3-day history of severe diarrhea, which causes profound gastrointestinal bicarbonate loss.

Step-by-Step Solution

1

Analyze the baseline arterial blood gas.

The baseline ABG shows a PaCO2 of 56 mm Hg and HCO3- of 31 mEq/L, consistent with a compensated chronic respiratory acidosis.

Establishing the patient's baseline is necessary to correctly interpret the current acute changes in patients with severe COPD.

2

Evaluate the current PaCO2 and calculate the expected compensation.

The PaCO2 has acutely increased by 10 mm Hg (from 56 to 66 mm Hg). The expected acute increase in HCO3- is 1 mEq/L, making the new expected HCO3- 32 mEq/L (31 + 1).

In acute respiratory acidosis, the HCO3- increases by 1 mEq/L for every 10 mm Hg increase in PaCO2.

3

Compare actual HCO3- to expected HCO3-.

The actual HCO3- is 23 mEq/L, which is significantly lower than the expected 32 mEq/L. This indicates a superimposed metabolic acidosis.

A lower-than-expected bicarbonate level in the setting of respiratory acidosis confirms a concurrent metabolic acidosis.

4

Calculate the anion gap.

Anion Gap = Na+ - (Cl- + HCO3-) = 135 - (102 + 23) = 10 mEq/L. This is within the normal range.

The anion gap differentiates between a normal anion gap and a high anion gap metabolic acidosis. The normal gap points to GI bicarbonate loss from diarrhea.

Key Concept

Patients with chronic respiratory acidosis who develop an acute exacerbation require step-wise acid-base analysis utilizing their baseline ABG values to accurately calculate expected compensation and identify superimposed metabolic disorders.

Estimated Time:2m 0s

Question 260Question →

Match each patient's acquired acid-base and electrolyte derangement profile with the pharmacological agent most likely responsible for these findings.

Click a left item, then click its matching right item

Show answer & explanation

Answer

The correct answer links the hypocalciuric alkalosis to thiazide diuretics, the hyperchloremic acidosis with alkaline urine to carbonic anhydrase inhibitors, the hypercalciuric alkalosis to loop diuretics, and the hyperkalemic acidosis to aldosterone antagonists.

Each pharmacological agent acts on a specific segment of the nephron, creating a unique signature of electrolyte and acid-base changes based on the localized transport mechanisms disrupted.

Step-by-Step Solution

1

Analyze the primary acid-base disturbance described in each presentation.

Two profiles exhibit metabolic alkalosis (patients 1 and 3), and two exhibit a normal anion gap metabolic acidosis (patients 2 and 4).

Categorizing the primary acid-base derangement immediately narrows down the physiological mechanism of action.

2

Differentiate the causes of metabolic alkalosis based on the urinary calcium handling.

Patient 1 presents with hypocalciuria (characteristic of thiazides), while Patient 3 presents with hypercalciuria (characteristic of loop diuretics).

Thiazides and loop diuretics both cause hypokalemic contraction alkalosis, but they have profoundly opposite effects on renal calcium handling.

3

Differentiate the causes of metabolic acidosis based on potassium levels and urine pH.

Patient 2 shows an alkaline urine pH due to bicarbonate wasting, identifying a carbonic anhydrase inhibitor. Patient 4 shows hyperkalemia, identifying an aldosterone antagonist.

Acetazolamide causes a proximal RTA-like picture with hypokalemia, whereas spironolactone causes a Type 4 RTA-like picture defined by hyperkalemia.

Key Concept

Diuretic-induced acid-base and electrolyte derangements can be precisely distinguished by their unique downstream effects on serum potassium, acid-base balance, and urinary calcium handling.

Estimated Time:1m 30s

Nephrology

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