Clinical Assessment of Acid-Base Status — MCQs

10 questions

All of the following statements about acid-base disorders are true, EXCEPT:

A 25-year-old male patient presents with ingestion of antifreeze solution. His arterial blood gas analysis report is as follows: pH = 7.20 Anion gap = 15 PCO2 = 25 HCO3 = 10 What is the most likely diagnosis?

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 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 person with type 1 diabetes ran out of her prescription insulin and has not been able to inject insulin for the past 3 days. The patient is hyperventilating to compensate for her metabolic acidosis. Which of the following reactions explains this respiratory compensation for metabolic acidosis?

Acid-base imbalance is suspected in a patient. Which of the following parameters would you use for initial determination of acid-base status?

Dosage of intravenous fluid for 2 month old child in diarrhea with severe dehydration -

What is the maximum limit of observation period for diagnosis of insanity?

Hereditary orotic aciduria Type-I is due to deficiency of?

Q10

An infant is brought by his parents with complaints that his urine turns black on standing. Which of the following metabolic disorders is likely?

Clinical Assessment of Acid-Base Status Indian Medical PG Practice Questions and MCQs

Question 1: All of the following statements about acid-base disorders are true, EXCEPT:

A. Metabolic acidosis is compensated by increasing Pco2 (Correct Answer)
B. Buffering may be intra & extra cellular
C. pH determined by Pco2 and HCO3
D. Respiratory acidosis is compensated by HCO3

Explanation: ***Metabolic acidosis is compensated by increasing Pco2*** - In **metabolic acidosis**, the primary problem is a decrease in **bicarbonate (HCO3-)**. - The compensatory response is **respiratory**, involving an increase in **respiratory rate** and depth to **decrease Pco2**, thereby *raising* the pH back towards normal. Increasing Pco2 would worsen the acidosis. *Buffering may be intra & extra cellular* - **Buffering systems** operate both **intracellularly** (e.g., proteins, phosphates) and **extracellularly** (e.g., bicarbonate-carbonic acid system, hemoglobin). - This dual buffering ensures a rapid and widespread response to changes in acid-base balance throughout the body. *pH determined by Pco2 and HCO3* - According to the **Henderson-Hasselbalch equation**, pH is directly proportional to the ratio of **bicarbonate (HCO3-)** to **Pco2**. - This means that changes in either Pco2 (respiratory component) or HCO3- (metabolic component) will directly influence the overall pH of the blood. *Respiratory acidosis is compensated by HCO3* - In **respiratory acidosis**, the primary problem is an increase in **Pco2** due to hypoventilation. - The compensatory response is **renal**, involving increased reabsorption of **bicarbonate (HCO3-)** and increased excretion of H+ ions to buffer the excess acid.

Question 2: A 25-year-old male patient presents with ingestion of antifreeze solution. His arterial blood gas analysis report is as follows: pH = 7.20 Anion gap = 15 PCO2 = 25 HCO3 = 10 What is the most likely diagnosis?

A. None of the options
B. Normal anion gap metabolic acidosis
C. High anion gap metabolic acidosis (Correct Answer)
D. Both

Explanation: ***High anion gap metabolic acidosis*** - The patient has a **low pH (7.20)**, indicating **acidosis**. The **bicarbonate (HCO3-) is low (10 mEq/L)**, which confirms it is a metabolic acidosis [1]. - The **anion gap is calculated as Na+ - (Cl- + HCO3-)**. With the given anion gap of 15, which is above the normal range (typically 8-12 mEq/L), it indicates a **high anion gap metabolic acidosis** [2]. This is consistent with **antifreeze (ethylene glycol) ingestion**, which produces acidic metabolites [2]. *Normal anion gap metabolic acidosis* - This type of acidosis occurs when the **anion gap remains within the normal range** (8-12 mEq/L), even though blood pH is low. - It usually results from a **loss of bicarbonate**, often through the gastrointestinal tract (e.g., severe diarrhea) or via the kidneys (e.g., renal tubular acidosis) [3], with a compensatory increase in chloride. *None of the options* - This option is incorrect as the presented clinical and lab findings clearly point to a specific type of acid-base disturbance. - The calculated anion gap and the pH/bicarbonate levels provide sufficient information for diagnosis. *Both* - This option is incorrect because the patient's lab values, specifically the **elevated anion gap**, distinctly categorize the condition as a high anion gap metabolic acidosis, ruling out a normal anion gap metabolic acidosis. - An acid-base disorder cannot simultaneously be both high and normal anion gap.

Question 3: 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 4: 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 5: A person with type 1 diabetes ran out of her prescription insulin and has not been able to inject insulin for the past 3 days. The patient is hyperventilating to compensate for her metabolic acidosis. Which of the following reactions explains this respiratory compensation for metabolic acidosis?

A. H2O ⇌ H+ + OH-
B. H+ + NH3 ⇌ NH4+
C. CH3CHOHCH2COOH ⇌ CH3CHOHCH2COO- + H+
D. CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3- (Correct Answer)

Explanation: ***CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3-*** - This reaction represents the **bicarbonate buffer system**, which is central to maintaining **pH balance** in the body. - In response to **metabolic acidosis**, the body hyperventilates to **decrease CO2** levels, shifting the equilibrium to the left and reducing H+ which compensates for the increased acidity. *H2O ⇌ H+ + OH-* - This reaction describes the **autoionization of water**, which is fundamental but does not directly explain the body's respiratory compensation mechanism for metabolic acidosis. - While it shows the presence of H+ ions, it doesn't illustrate how the respiratory system manipulates CO2 to influence pH. *H+ + NH3 ⇌ NH4+* - This reaction represents the **ammonia buffer system** primarily active in the **kidneys** for acid excretion. - It plays a role in renal compensation for pH imbalances, but it is not the mechanism for respiratory compensation. *CH3CHOHCH2COOH ⇌ CH3CHOHCH2COO- + H+* - This represents the **dissociation of beta-hydroxybutyric acid**, a **ketone body** produced in diabetic ketoacidosis (DKA). - While DKA is the cause of the metabolic acidosis in this patient, this specific reaction describes the *production* of H+ ions, not the *respiratory compensatory mechanism* to address it.

Question 6: Acid-base imbalance is suspected in a patient. Which of the following parameters would you use for initial determination of acid-base status?

A. pH, PaCO2, and Base excess
B. pH, PaCO2, and Bicarbonate (Correct Answer)
C. pH and PaCO2
D. pH, PaCO2, Bicarbonate, and Base excess

Explanation: ***pH, PaCO2, and Bicarbonate*** - The **pH** provides immediate assessment of overall acid-base status (acidemia if <7.35 or alkalemia if >7.45) - The **PaCO2** reflects the respiratory component - elevated in respiratory acidosis or compensated metabolic alkalosis; decreased in respiratory alkalosis or compensated metabolic acidosis - The **HCO3- (bicarbonate)** reflects the metabolic component - essential for determining whether the primary disorder is metabolic or respiratory - This triad forms the **standard approach** to arterial blood gas (ABG) interpretation taught in all major medical textbooks - Together, these three parameters allow complete initial classification of acid-base disorders using the Henderson-Hasselbalch relationship *pH and PaCO2* - While pH and PaCO2 are critical measurements, **without bicarbonate**, you cannot differentiate between metabolic and respiratory disorders or assess metabolic compensation - For example, a low pH with normal PaCO2 could indicate metabolic acidosis, but you need HCO3- to confirm this diagnosis - Incomplete for initial acid-base determination *pH, PaCO2, and Base excess* - Base excess is a **calculated parameter** used to quantify the metabolic component of acid-base disturbances - While useful, it is considered a **secondary parameter** for more detailed metabolic analysis rather than essential for initial determination - Standard ABG interpretation uses bicarbonate, not base excess, as the primary metabolic parameter *pH, PaCO2, Bicarbonate, and Base excess* - While this includes all relevant parameters, **base excess is redundant** for initial determination - Base excess adds quantitative information about metabolic component but is not required for the initial classification of acid-base status - The essential triad for initial assessment is pH, PaCO2, and HCO3-

Question 7: Dosage of intravenous fluid for 2 month old child in diarrhea with severe dehydration -

A. 80 ml/Kg in 6 hour
B. 50 ml/Kg in 6 hour
C. 100 ml/Kg in 6 hour (Correct Answer)
D. 75 ml/Kg in 6 hour

Explanation: ***100 ml/Kg in 6 hour*** - For infants under 12 months with **severe dehydration** due to diarrhea, the standard recommendation for intravenous fluid resuscitation is to administer **100 ml/kg** over 6 hours. - This volume is divided, with 30 mL/kg given in the first hour, and the remaining 70 mL/kg given over the subsequent 5 hours, following the **WHO guidelines** for rehydration. *80 ml/Kg in 6 hour* - This dosage is **insufficient** for severe dehydration in infants, as it would not adequately replace the significant fluid and electrolyte deficits. - Undershooting the fluid requirements in severe dehydration can lead to persistent **hypovolemic shock** and worsen clinical outcomes. *50 ml/Kg in 6 hour* - This is a **critically low dose** for severe dehydration and would be entirely inadequate for effective rehydration in a 2-month-old. - Such a low fluid volume would fail to correct **circulatory compromise** and could lead to rapid clinical deterioration. *75 ml/Kg in 6 hour* - While closer to the recommended dose than other incorrect options, **75 ml/kg** is still generally considered insufficient for a 2-month-old with severe dehydration. - This dose may be appropriate for **less severe dehydration** or if fluid therapy is initiated too slowly, putting the infant at risk of incomplete rehydration.

Question 8: What is the maximum limit of observation period for diagnosis of insanity?

A. 50 days
B. 30 days (Correct Answer)
C. 5 days
D. 10 days

Explanation: ***30 days*** - The maximum observation period for diagnosing **insanity** in a legal context is typically set at **30 days**. - This period allows for sufficient time to conduct thorough psychiatric evaluations, observe behavior, and gather collateral information to establish the presence or absence of a **mental disorder** at the time of the alleged offense. *50 days* - An observation period of **50 days** is excessively long for routine determination of insanity in most legal jurisdictions. - While prolonged observation might occur in complex or unusual cases, it is not the standard maximum limit. *5 days* - A **5-day observation period** is generally too short to reliably diagnose or rule out **insanity**. - Many mental health conditions may not manifest consistently within such a brief timeframe, leading to potentially inaccurate assessments. *10 days* - **10 days** is also considered an insufficient period for a comprehensive evaluation of **insanity**. - It may not provide enough opportunity to observe fluctuations in mental state or to gather all necessary diagnostic information.

Question 9: Hereditary orotic aciduria Type-I is due to deficiency of?

A. Orotate phosphoribosyl transferase
B. UMP synthase (Correct Answer)
C. Orotic acid decarboxylase
D. All of the options

Explanation: ***UMP synthase*** - Hereditary orotic aciduria Type-I is caused by a deficiency of the **bifunctional enzyme UMP synthase** (also called UMP synthase complex). - UMP synthase catalyzes two sequential reactions in the *de novo* pyrimidine synthesis pathway: 1. **OPRT activity**: Converts orotate → orotidine 5'-monophosphate (OMP) 2. **ODC activity**: Converts OMP → uridine 5'-monophosphate (UMP) - This is the **most precise and complete answer** as it identifies the actual enzyme complex that is deficient. - **Clinical features**: Megaloblastic anemia, growth retardation, immunodeficiency; responds to oral uridine supplementation. *Orotate phosphoribosyl transferase* - This represents only **one of the two catalytic activities** of the UMP synthase enzyme (the first step). - While this activity is indeed deficient in Type-I orotic aciduria, naming only this activity is **incomplete** because the enzyme has two functions. - This would be a **partial answer** rather than the complete enzyme name. *Orotic acid decarboxylase* - This represents only **the second catalytic activity** of the UMP synthase enzyme (converts OMP to UMP). - Like OPRT, this activity is also deficient, but naming only this component is **incomplete**. - **Type II orotic aciduria** (extremely rare) involves isolated ODC deficiency without OPRT deficiency. *All of the options* - While technically both OPRT and ODC activities are affected in Type-I orotic aciduria, the **standard nomenclature** refers to the deficient enzyme as **"UMP synthase"** - the name of the complete bifunctional enzyme. - In medical terminology and examination context, we identify enzyme deficiencies by the **name of the enzyme complex**, not by listing all its individual catalytic activities. - Therefore, **"UMP synthase"** is the single most accurate and complete answer.

Question 10: An infant is brought by his parents with complaints that his urine turns black on standing. Which of the following metabolic disorders is likely?

A. Phenylketonuria
B. Alkaptonuria (Correct Answer)
C. Homocystinuria
D. Maple syrup urine disease

Explanation: ***Alkaptonuria*** - **Alkaptonuria** is an autosomal recessive disorder characterized by a deficiency of **homogentisate 1,2-dioxygenase**, an enzyme involved in the metabolism of tyrosine. - The accumulation of **homogentisic acid** in tissues and urine causes the urine to turn black on standing due to oxidation. *Phenylketonuria* - **Phenylketonuria (PKU)** is caused by a deficiency of **phenylalanine hydroxylase**, leading to the accumulation of phenylalanine. - While it can manifest with intellectual disability and neurological symptoms, it does not typically cause the urine to turn black. *Homocystinuria* - **Homocystinuria** is a disorder of methionine metabolism, typically due to a deficiency of **cystathionine beta-synthase**. - It is characterized by intellectual disability, skeletal abnormalities, and lens dislocation, but not black urine. *Maple syrup urine disease* - **Maple syrup urine disease (MSUD)** results from a deficiency of **branched-chain alpha-keto acid dehydrogenase complex**, leading to the accumulation of branched-chain amino acids. - The distinguishing feature is urine that smells like maple syrup, not turning black.

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