Respiratory and Metabolic Acidosis — MCQs
In a patient with a pH of 7.2 and a bicarbonate level of 15 mEq/L, what does this imply about the acid-base status?
High anion gap acidosis is seen in all except:
Which of the following is a cause of metabolic acidosis with a normal anion gap?
In which of the following condition normal anion gap metabolic acidosis is seen?
Renal tubular acidosis with ABG value pH = 7.24 PO2=80; PaCO2= 36 Na = 131; HCO3 = 14 Cl= 90; BE = -13 Glucose = 135 the above ABG picture suggests –
A patient presents with the following arterial blood gas (ABG) and electrolyte values: pH: 7.34, Na: 135 mEq/L, Cl: 93 mEq/L, HCO3: 20 mEq/L, Random Blood Sugar (RBS): 420 mg/dl. What is the most likely acid-base disturbance?
A male patient presents to the emergency department. The arterial blood gas report is as follows: pH, 7.2; pCO2, 81 mmHg; and HCO3, 40 meq/L. Which of the following is the most likely diagnosis?
In plasma, if pH is 5, what is the fraction of base to acid?
In metabolic acidosis, what compensatory mechanism is activated first?
In severe exercise, decrease in pH is due to
Respiratory and Metabolic Acidosis Indian Medical PG Practice Questions and MCQs
Question 1: In a patient with a pH of 7.2 and a bicarbonate level of 15 mEq/L, what does this imply about the acid-base status?
- A. Normal acid-base status
- B. Respiratory acidosis with partial metabolic compensation
- C. Respiratory acidosis
- D. Metabolic acidosis (Correct Answer)
Explanation: ***Metabolic acidosis*** - A **low pH (7.2)** indicates **acidosis**, and a **low bicarbonate level (15 mEq/L)** is the primary disturbance suggesting a metabolic origin. - This combination points to an excess of acid or a loss of bicarbonate, leading to **metabolic acidosis**. *Normal acid-base status* - A **normal pH** would typically range from 7.35 to 7.45, and **bicarbonate** levels would be between 22-26 mEq/L. - The given values (pH 7.2, HCO3 15 mEq/L) are significantly outside of these normal ranges. *Respiratory acidosis with partial metabolic compensation* - Respiratory acidosis is characterized by a **low pH** and an **elevated pCO2**, not primarily a low bicarbonate. - Metabolic compensation would involve an increase in bicarbonate to buffer the acidosis, but the primary disturbance here is low bicarbonate, indicating a metabolic rather than respiratory etiology. *Respiratory acidosis* - Respiratory acidosis is primarily caused by **hypoventilation**, leading to an **increase in pCO2** and a decrease in pH. - In this case, the primary abnormality is the **low bicarbonate**, not an elevated pCO2, which characterizes metabolic acidosis.
Question 2: High anion gap acidosis is seen in all except:
- A. Diarrhea (Correct Answer)
- B. Salicylate poisoning
- C. Acute renal failure
- D. Lactic acidosis
Explanation: ***Diarrhea*** - Diarrhea causes **non-anion gap metabolic acidosis** due to the loss of bicarbonate from the gastrointestinal tract [3]. - The anion gap remains normal because chloride reabsorption in the kidneys increases to compensate for the lost bicarbonate. *Salicylate poisoning* - Salicylate poisoning causes **high anion gap metabolic acidosis** by uncoupling oxidative phosphorylation, leading to increased production of lactic acid and other organic acids [2]. - Early stages may also involve a superimposed respiratory alkalosis due to direct stimulation of the respiratory center [1]. *Acute renal failure* - Acute renal failure leads to a **high anion gap metabolic acidosis** because the kidneys are unable to excrete metabolic acids (e.g., phosphates, sulfates) and reabsorb bicarbonate effectively [3]. - This results in the accumulation of unmeasured anions and a decrease in serum bicarbonate. *Lactic acidosis* - Lactic acidosis is a common cause of **high anion gap metabolic acidosis**, resulting from increased production or decreased metabolism of lactic acid [1]. - It occurs when there is inadequate tissue oxygenation (Type A, e.g., shock) or other conditions like certain drugs or toxins (Type B) [1].
Question 3: Which of the following is a cause of metabolic acidosis with a normal anion gap?
- A. Diabetic ketoacidosis
- B. Aspirin poisoning
- C. Renal tubular acidosis (Correct Answer)
- D. Lactic acidosis
Explanation: ***Renal tubular acidosis*** - **Renal tubular acidosis (RTA)** is characterized by a defect in renal acid excretion or bicarbonate reabsorption, leading to **metabolic acidosis** with a **normal anion gap** [1]. - The deficiency in net acid excretion results in the retention of chloride ions to maintain electroneutrality, hence it's also known as **hyperchloremic metabolic acidosis** [1]. *Diabetic ketoacidosis* - **Diabetic ketoacidosis (DKA)** is a high anion gap metabolic acidosis caused by the accumulation of **ketoacids** (beta-hydroxybutyrate, acetoacetate). - These unmeasured anions increase the anion gap, distinguishing it from normal anion gap acidosis. *Aspirin poisoning* - **Aspirin (salicylate) poisoning** typically causes a **mixed acid-base disorder** with both metabolic acidosis and respiratory alkalosis [1]. - The metabolic acidosis component is a **high anion gap acidosis** due to the accumulation of salicylates and their metabolites. *Lactic acidosis* - **Lactic acidosis** is a common cause of **high anion gap metabolic acidosis**, resulting from the overproduction or decreased clearance of **lactate** [1]. - The increased concentration of lactate, an unmeasured anion, leads to the widening of the anion gap.
Question 4: In which of the following condition normal anion gap metabolic acidosis is seen?
- A. Lactic acidosis
- B. Diabetic ketoacidosis
- C. Diarrhoea (Correct Answer)
- D. Renal failure
Explanation: ***Diarrhoea*** - Diarrhoea causes a loss of **bicarbonate-rich fluid** from the gastrointestinal tract [2]. - This loss leads to an increase in **serum chloride** to maintain electroneutrality, resulting in a normal anion gap metabolic acidosis. *Lactic acidosis* - Lactic acidosis results from the overproduction or under-elimination of **lactic acid** [1]. - Lactic acid is an **unmeasured anion**, leading to an **increased anion gap** metabolic acidosis. *Diabetic ketoacidosis* - Diabetic ketoacidosis involves the accumulation of **ketone bodies** (beta-hydroxybutyrate, acetoacetate), which are unmeasured anions [2]. - This accumulation causes an **increased anion gap** metabolic acidosis. *Renal failure* - Chronic renal failure can cause metabolic acidosis through the retention of **phosphate** and **sulfate**, which are unmeasured anions [2]. - This typically results in an **increased anion gap** metabolic acidosis, although some forms of renal tubular acidosis can cause a normal anion gap [1].
Question 5: Renal tubular acidosis with ABG value pH = 7.24 PO2=80; PaCO2= 36 Na = 131; HCO3 = 14 Cl= 90; BE = -13 Glucose = 135 the above ABG picture suggests –
- A. Metabolic acidosis (Correct Answer)
- B. Respiratory alkalosis
- C. Metabolic alkalosis
- D. Respiratory acidosis
Explanation: The ABG shows a pH of 7.24, indicating **acidemia** [1]. The HCO3 is 14 mEq/L, which is significantly **low**, and the base excess (BE) is -13 [1]. The PaCO2 of 36 mmHg is within the normal range, indicating no significant primary respiratory derangement [2]. The overall picture is consistent with an uncompensated or partially compensated **metabolic acidosis** [1][2]. ***Metabolic acidosis*** - The **low pH (acidemia)**, **low bicarbonate (HCO3)**, and **negative base excess (BE)** are direct indicators of metabolic acidosis [1]. - The **PaCO2 within normal limits** or slightly decreased suggests either no respiratory compensation or insufficient compensation for the metabolic derangement [1][2]. *Respiratory acidosis* - This would present with a **low pH** and an **elevated PaCO2** as the primary defect, which is not seen here (PaCO2 is normal) [1]. - Bicarbonate would typically be normal or elevated if compensated, not significantly decreased. *Respiratory alkalosis* - This would be characterized by an **elevated pH** and a **low PaCO2**, which is the opposite of the findings in this ABG [1]. - HCO3 would be normal or low if compensated. *Metabolic alkalosis* - This would present with an **elevated pH** and an **elevated HCO3**, which contradicts the given ABG values (low pH and low HCO3) [2].
Question 6: A patient presents with the following arterial blood gas (ABG) and electrolyte values: pH: 7.34, Na: 135 mEq/L, Cl: 93 mEq/L, HCO3: 20 mEq/L, Random Blood Sugar (RBS): 420 mg/dl. What is the most likely acid-base disturbance?
- A. Normal Anion Gap Metabolic Acidosis (NAGMA)
- B. Respiratory Acidosis
- C. High Anion Gap Metabolic Acidosis (HAGMA) (Correct Answer)
- D. Metabolic Alkalosis
Explanation: ### High Anion Gap Metabolic Acidosis (HAGMA) - The **pH (7.34)** indicates **acidemia**, and the **low bicarbonate (20 mEq/L)** suggests a metabolic acidosis [1], [2]. - Calculation of the anion gap: Na - (Cl + HCO3) = 135 - (93 + 20) = 22 mEq/L. An anion gap > 12 mEq/L is considered high, confirming **High Anion Gap Metabolic Acidosis (HAGMA)** [4]. The **RBS of 420 mg/dl** also points towards a likely cause such as **diabetic ketoacidosis** [3]. *Normal Anion Gap Metabolic Acidosis (NAGMA)* - This would be present if the calculated anion gap were within the normal range (typically 8-12 mEq/L). - Causes of NAGMA (e.g., hyperchloremic acidosis) are typically associated with increased chloride levels to compensate for the bicarbonate loss, which is not the primary finding here [4]. *Respiratory Acidosis* - This condition is characterized by a **low pH** and an **elevated PaCO2**, which is not provided but implied by the **low bicarbonate** not fitting a respiratory picture [2]. - While the pH is low, the primary disturbance given the other values (especially the low bicarbonate) is metabolic, not respiratory. *Metabolic Alkalosis* - Metabolic alkalosis is characterized by an **elevated pH** and an **elevated bicarbonate level**, which contradicts the presented values of low pH and low bicarbonate [2]. - This condition would involve a net gain of bicarbonate or a loss of acids, which is the opposite of the findings in this patient.
Question 7: A male patient presents to the emergency department. The arterial blood gas report is as follows: pH, 7.2; pCO2, 81 mmHg; and HCO3, 40 meq/L. Which of the following is the most likely diagnosis?
- A. Respiratory alkalosis
- B. Metabolic acidosis
- C. Respiratory acidosis (Correct Answer)
- D. Metabolic alkalosis
Explanation: ***Respiratory acidosis*** - The **pH of 7.2** indicates **acidemia**, while the **elevated pCO2 (81 mmHg)** points to a primary respiratory problem [2]. - The elevated **HCO3 (40 meq/L)** suggests **renal compensation** attempting to buffer the increased carbonic acid [1]. *Respiratory alkalosis* - This condition presents with an **elevated pH (alkalemia)** and a **decreased pCO2**, which is opposite to the given ABG values [2]. - While there might be metabolic compensation with a decreased HCO3, the primary disturbance is an increase in respiratory rate leading to excessive CO2 exhalation. *Metabolic acidosis* - Metabolic acidosis is characterized by a **low pH** and a **low HCO3**, with a compensatory decrease in pCO2 [1]. - The given ABG shows a high HCO3, which rules out primary metabolic acidosis. *Metabolic alkalosis* - This condition would typically show an **elevated pH** and an **elevated HCO3**, with a compensatory increase in pCO2. - While both HCO3 and pCO2 are high in the given ABG, the low pH points to a primary acidosis, not alkalosis.
Question 8: In plasma, if pH is 5, what is the fraction of base to acid?
- A. 0.01
- B. 0.1 (Correct Answer)
- C. 1
- D. 10
Explanation: ***0.1*** - This question applies the **Henderson-Hasselbalch equation**: pH = pKa + log([base]/[acid]). For the **bicarbonate buffer system** (the primary plasma buffer), pKa ≈ 6.1. - Substituting the given values: $5 = 6.1 + \log([HCO_3^-] / [H_2CO_3])$ - Rearranging: $\log([HCO_3^-] / [H_2CO_3]) = 5 - 6.1 = -1.1$ - Therefore: $[HCO_3^-] / [H_2CO_3] = 10^{-1.1} ≈ 0.079$ - Among the given options, **0.079 is closest to 0.1**, making this the correct answer. - Note: pH 5 in plasma is physiologically impossible (incompatible with life), but this tests theoretical understanding of the buffer equation. *0.01* - This ratio would correspond to an even **more acidic** condition with $\log([base]/[acid]) = -2$. - Using Henderson-Hasselbalch: pH = 6.1 + (-2) = 4.1, which is lower than the given pH of 5. - The calculated ratio of 0.079 is much closer to 0.1 than to 0.01. *1* - A ratio of 1 means **equal concentrations** of base and acid, which occurs when pH = pKa. - This would give pH = 6.1, not the given pH of 5. - This represents a **neutral buffer condition**, not the acidic state described. *10* - This ratio indicates an **alkaline** solution with 10 times more base than acid. - Using Henderson-Hasselbalch: pH = 6.1 + log(10) = 6.1 + 1 = 7.1 (physiological alkalosis). - This contradicts the given acidic pH of 5.
Question 9: In metabolic acidosis, what compensatory mechanism is activated first?
- A. Decreased CO2 excretion
- B. Increased respiratory rate (Correct Answer)
- C. Increased renal HCO3- excretion
- D. Increased renal H+ secretion
Explanation: ***Increased respiratory rate*** - In metabolic acidosis, the body attempts to **decrease PCO2** through increasing ventilation, thus reducing the **acid load** by expelling more CO2. - This **respiratory compensation** is rapid and begins within minutes to hours of the onset of acidosis. *Decreased CO2 excretion* - This option is incorrect because the body's compensatory mechanism for acidosis involves **increasing CO2 excretion** through hyperventilation, not decreasing it. - Decreased CO2 excretion would lead to **respiratory acidosis**, further worsening the metabolic acidosis. *Increased renal HCO3- excretion* - In metabolic acidosis, the kidneys aim to **conserve bicarbonate**, not excrete it, to buffer the excess acid. - Increased renal HCO3- excretion would exacerbate acidosis by reducing the body's primary **buffer system**. *Increased renal H+ secretion* - This is a renal compensatory mechanism that occurs in metabolic acidosis, but it is **slower to activate** (hours to days) compared to respiratory compensation. - While important for long-term acid-base balance, it is **not the first mechanism** to be activated.
Question 10: In severe exercise, decrease in pH is due to
- A. Lactic acidosis (Correct Answer)
- B. H+ retention
- C. Respiratory acidosis
- D. HCO3- excretion
Explanation: ***Lactic acidosis*** - During severe exercise, muscles switch to **anaerobic metabolism** due to insufficient oxygen supply, leading to the production of **lactic acid**. - Lactic acid dissociates into lactate and **hydrogen ions (H+)**, increasing H+ concentration and thus decreasing pH. *H+ retention* - **H+ retention** is a generalized term for metabolic acidosis, but it doesn't specifically explain the physiological mechanism during severe exercise. - The primary cause for increased H+ in this context is the production of lactic acid, not just generic retention. *Respiratory acidosis* - **Respiratory acidosis** occurs due to hypoventilation, leading to an accumulation of carbon dioxide (CO2) and a decrease in pH. - During severe exercise, the body typically hyperventilates to blow off CO2, which would counteract, not cause, acidosis. *HCO3- excretion* - **Bicarbonate (HCO3-) excretion** is a renal mechanism that helps regulate acid-base balance, primarily in response to metabolic alkalosis or to compensate for chronic acidosis. - While the kidneys might eventually excrete HCO3- to compensate for chronic acidosis, it is not the primary cause of acute pH decrease during severe exercise.
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