Henderson-Hasselbalch equation US Medical PG Practice Questions and MCQs
Practice US Medical PG questions for Henderson-Hasselbalch equation. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
Henderson-Hasselbalch equation US Medical PG Question 1: Which region of the nephron reabsorbs the highest percentage of filtered bicarbonate?
- A. Collecting duct
- B. Thick ascending limb
- C. Distal tubule
- D. Proximal tubule (Correct Answer)
Henderson-Hasselbalch equation Explanation: ***Proximal tubule***
- The **proximal convoluted tubule (PCT)** reabsorbs approximately 80-90% of the **filtered bicarbonate** through a process involving **carbonic anhydrase** and the **Na+/H+ exchanger**.
- This vital function ensures that the majority of bicarbonate, a key buffer, is returned to the blood to maintain **acid-base balance**.
*Collecting duct*
- While the collecting duct does have the ability to reabsorb and secrete bicarbonate, its contribution is minor compared to the PCT, primarily for fine-tuning acid-base balance.
- Cells in the collecting duct, particularly **Type A intercalated cells**, are important for secreting acid (H+) in acidosis and therefore reabsorbing bicarbonate, but not the bulk of it.
*Thick ascending limb*
- The primary role of the **thick ascending limb** is the reabsorption of **sodium**, **potassium**, and **chloride** to create a concentrated interstitium, not significant bicarbonate reabsorption.
- It is largely impermeable to water and is relatively impermeable to bicarbonate.
*Distal tubule*
- The **distal convoluted tubule (DCT)** reabsorbs a small percentage of filtered bicarbonate, but its main role is regulated reabsorption of **sodium** and **calcium**, and secretion of **potassium** and **hydrogen ions**.
- Its contribution to bicarbonate reabsorption is much less significant than that of the proximal tubule.
Henderson-Hasselbalch equation US Medical PG Question 2: In your peripheral tissues and lungs, carbonic anhydrase works to control the equilibrium between carbon dioxide and carbonic acid in order to maintain proper blood pH. Through which mechanism does carbonic anhydrase exert its influence on reaction kinetics?
- A. Changes the delta G of the reaction
- B. Lowers the free energy of products
- C. Raises the activation energy
- D. Lowers the activation energy (Correct Answer)
- E. Lowers the free energy of reactants
Henderson-Hasselbalch equation Explanation: ***Lowers the activation energy***
- Enzymes like **carbonic anhydrase** function by providing an alternative reaction pathway with a **lower activation energy**.
- This reduction in activation energy leads to a significant increase in the **reaction rate**, allowing the reaction to proceed more quickly without altering its equilibrium.
*Changes the delta G of the reaction*
- Enzymes do not alter the **overall Gibbs free energy change (ΔG)** of a reaction.
- The **ΔG** is a thermodynamic property determined by the difference in free energy between reactants and products, which is unaffected by catalyst.
*Lowers the free energy of products*
- Enzymes do not change the **free energy** of either the products or the reactants.
- Affecting the free energy of products would alter the overall **ΔG** of the reaction and thus the **equilibrium constant**.
*Raises the activation energy*
- Raising the **activation energy** would slow down the reaction rate, which is the opposite of an enzyme's function.
- Enzymes are catalysts designed to **accelerate reactions** by lowering the energy barrier.
*Lowers the free energy of reactants*
- Enzymes do not change the **free energy** of reactants.
- Altering the free energy of reactants would also change the overall **ΔG** of the reaction and its equilibrium.
Henderson-Hasselbalch equation US Medical PG Question 3: A 52-year-old man with a history of Type 1 diabetes mellitus presents to the emergency room with increasing fatigue. Two days ago, he ran out of insulin and has not had time to obtain a new prescription. He denies fevers or chills. His temperature is 37.2 degrees Celsius, blood pressure 84/56 mmHg, heart rate 100/min, respiratory rate 20/min, and SpO2 97% on room air. His physical exam is otherwise within normal limits. An arterial blood gas analysis shows the following:
pH 7.25, PCO2 29, PO2 95, HCO3- 15.
Which of the following acid-base disorders is present?
- A. Respiratory alkalosis with appropriate metabolic compensation
- B. Respiratory acidosis with appropriate metabolic compensation
- C. Mixed metabolic and respiratory acidosis
- D. Metabolic acidosis with appropriate respiratory compensation (Correct Answer)
- E. Metabolic alkalosis with appropriate respiratory compensation
Henderson-Hasselbalch equation Explanation: ***Metabolic acidosis with appropriate respiratory compensation***
- The patient's pH of 7.25 and HCO3- of 15 indicate **metabolic acidosis**, while the PCO2 of 29 indicates **respiratory compensation**.
- The compensation is **appropriate** as suggested by Winter's formula [Expected PCO2 = (1.5 x HCO3-) + 8 +/- 2; (1.5 x 15) + 8 = 30.5, which is close to 29].
*Respiratory alkalosis with appropriate metabolic compensation*
- This would involve a **pH > 7.45** and **low PCO2** with a secondary drop in HCO3-, which is not seen here.
- The patient's primary problem is a metabolic disturbance due to insulin deficiency.
*Respiratory acidosis with appropriate metabolic compensation*
- This disorder is characterized by a **low pH** and a **high PCO2**, with a secondary rise in HCO3-.
- The patient's PCO2 is low, indicating a compensatory response rather than a primary respiratory acidosis.
*Mixed metabolic and respiratory acidosis*
- A mixed disorder would show a **low pH** due to both **low HCO3-** and **high PCO2**.
- The patient's PCO2 is low, indicating a compensatory response to metabolic acidosis, not an additional respiratory acidosis.
*Metabolic alkalosis with appropriate respiratory compensation*
- This would present with a **high pH (>7.45)** and **high HCO3-**, with compensatory **elevated PCO2**.
- The patient's pH and HCO3- are low, indicating acidosis, not alkalosis.
Henderson-Hasselbalch equation US Medical PG Question 4: A 37-year-old G1P0 woman presents to her primary care physician for a routine checkup. She has a history of diabetes and hypertension but has otherwise been healthy with no change in her health status since the last visit. She is expecting her first child 8 weeks from now. She also enrolled in a study about pregnancy where serial metabolic panels and arterial blood gases are obtained. Partial results from these studies are shown below:
Serum:
Na+: 141 mEq/L
Cl-: 108 mEq/L
pH: 7.47
pCO2: 30 mmHg
HCO3-: 21 mEq/L
Which of the following disease processes would most likely present with a similar panel of metabolic results?
- A. Diarrheal disease
- B. Loop diuretic abuse
- C. Living at high altitude (Correct Answer)
- D. Ingestion of metformin
- E. Anxiety attack
Henderson-Hasselbalch equation Explanation: ***Living at high altitude***
- Chronic exposure to **high altitude** leads to sustained **hypoxia**, which stimulates **hyperventilation** as a compensatory mechanism.
- This persistent hyperventilation causes a **respiratory alkalosis** (high pH, low pCO2) and a compensatory **metabolic acidosis** (low HCO3-) to normalize pH, mimicking the presented metabolic panel.
*Diarrheal disease*
- Severe **diarrhea** leads to the loss of bicarbonate from the gastrointestinal tract, causing a **non-anion gap metabolic acidosis**.
- This would present with a **low pH**, **low HCO3-**, and a **compensatory drop in pCO2**, not a respiratory alkalosis with a high pH.
*Loop diuretic abuse*
- Chronic abuse of **loop diuretics** can cause **metabolic alkalosis** due to increased renal excretion of hydrogen ions and potassium, leading to volume contraction.
- This would typically present with a **high pH**, high HCO3-, and a compensatory rise in pCO2, which is different from the given values.
*Ingestion of metformin*
- **Metformin** can cause **lactic acidosis** (a type of high anion gap metabolic acidosis), especially in patients with renal impairment.
- This would manifest as a **low pH**, **low HCO3-**, and a **compensatory decrease in pCO2**, along with an elevated anion gap, not the respiratory alkalosis seen here.
*Anxiety attack*
- An **anxiety attack** causes acute **hyperventilation**, leading to **acute respiratory alkalosis** (high pH, low pCO2).
- However, in an acute setting, there is insufficient time for significant renal compensation, so the HCO3- would remain near normal, unlike the compensated state shown in the panel.
Henderson-Hasselbalch equation US Medical PG Question 5: A 25-year-old woman with an extensive psychiatric history is suspected of having metabolic acidosis after ingesting a large amount of aspirin in a suicide attempt. Labs are drawn and the values from the ABG are found to be: PCO2: 25, and HCO3: 15, but the pH value is smeared on the print-out and illegible. The medical student is given the task of calculating the pH using the pCO2 and HCO3 concentrations. He recalls from his first-year physiology course that the pKa of relevance for the bicarbonate buffering system is approximately 6.1. Which of the following is the correct formula the student should use, using the given values from the incomplete ABG?
- A. 15/6.1 + log[10/(0.03*25)]
- B. 6.1 + log[15/(0.03*25)] (Correct Answer)
- C. 10^6.1 + 15/0.03*25
- D. 6.1 + log[0.03/15*25]
- E. 6.1 + log[25/(15*0.03)]
Henderson-Hasselbalch equation Explanation: ***6.1 + log[15/(0.03*25)]***
- This formula correctly represents the Henderson-Hasselbalch equation for the bicarbonate buffer system: **pH = pKa + log([HCO3-]/[0.03 * PCO2])**.
- Here, **pKa is 6.1**, **[HCO3-] is 15**, and **[0.03 * PCO2] is 0.03 * 25**, making this the appropriate calculation for pH.
*15/6.1 + log[10/(0.03*25)]*
- This formula incorrectly places the pKa in the denominator of the first term and introduces an arbitrary '10' in the numerator of the logarithmic term.
- The **Henderson-Hasselbalch equation** dictates that pKa is added, not divided into, another component, and the logarithmic term should reflect the ratio of bicarbonate to carbonic acid.
*10^6.1 + 15/0.03*25*
- This option incorrectly uses an exponentiation of pKa and adds it to an unrelated fractional term, which does not correspond to the Henderson-Hasselbalch equation structure.
- The formula for pH calculation is a sum of pKa and a logarithmic term, not an exponentiation and a simple fraction.
*6.1 + log[0.03/15*25]*
- This option incorrectly inverts the ratio within the logarithm, placing the carbonic acid component (0.03 * PCO2) in the numerator and bicarbonate in the denominator.
- The correct Henderson-Hasselbalch equation requires the **bicarbonate concentration in the numerator** and the carbonic acid concentration in the denominator.
*6.1 + log [25/(15*0.03)]*
- This option incorrectly places the PCO2 (25) in the numerator of the logarithmic term and the product of HCO3- and 0.03 in the denominator.
- The correct ratio for the Henderson-Hasselbalch equation is **[HCO3-] / [0.03 * PCO2]**.
Henderson-Hasselbalch equation US Medical PG Question 6: A 24-year-old male is brought in by ambulance to the emergency department after he was found unresponsive at home for an unknown length of time. Upon arrival, he is found to be severely altered and unable to answer questions about his medical history. Based on clinical suspicion, a panel of basic blood tests are obtained including an arterial blood gas, which shows a pH of 7.32, a pCO2 of 70, and a bicarbonate level of 30 mEq/L. Which of the following is most likely the primary disturbance leading to the values found in the ABG?
- A. Respiratory acidosis (Correct Answer)
- B. Metabolic alkalosis
- C. Respiratory alkalosis
- D. Metabolic acidosis
- E. Mixed alkalosis
Henderson-Hasselbalch equation Explanation: ***Respiratory acidosis***
- The **pH (7.32)** is acidic (normal 7.35-7.45), and the **pCO2 (70 mmHg)** is significantly elevated (normal 35-45 mmHg), indicating **primary respiratory acidosis** due to hypoventilation.
- The **bicarbonate (30 mEq/L)** is elevated above normal (22-26 mEq/L), indicating **partial metabolic compensation** by the kidneys retaining bicarbonate to buffer the acidosis.
- This pattern suggests **chronic respiratory acidosis** (e.g., from COPD, CNS depression, neuromuscular disease) with renal compensation.
*Metabolic alkalosis*
- This would present with **elevated pH** (>7.45) and **elevated bicarbonate** as the primary disturbance, often with compensatory elevation in pCO2.
- The patient's **pH is acidic (7.32)**, not alkalotic, ruling out metabolic alkalosis as the primary process.
*Respiratory alkalosis*
- This would present with **elevated pH** (>7.45) and **decreased pCO2** (<35 mmHg) due to hyperventilation.
- The patient has the opposite: **acidic pH and elevated pCO2**, ruling out respiratory alkalosis.
*Metabolic acidosis*
- This would present with **decreased pH** and **decreased bicarbonate** (<22 mEq/L) as the primary disturbance.
- While the pH is low, the **bicarbonate is elevated (30 mEq/L)**, not decreased, ruling out metabolic acidosis as the primary disorder.
*Mixed alkalosis*
- A mixed alkalosis would involve simultaneous respiratory and metabolic processes causing **elevated pH**.
- The patient's **pH is acidic (7.32)**, making any form of alkalosis impossible as the primary disturbance.
Henderson-Hasselbalch equation US Medical PG Question 7: A 32-year-old female with Crohn's disease diagnosed in her early 20s comes to your office for a follow-up appointment. She is complaining of headaches and fatigue. Which of the following arterial blood gas findings might you expect?
- A. High PaO2, normal O2 saturation (SaO2), normal O2 content (CaO2)
- B. Low PaO2, low O2 saturation (SaO2), low O2 content (CaO2)
- C. Normal PaO2, normal O2 saturation (SaO2), normal O2 content (CaO2)
- D. Normal PaO2, normal O2 saturation (SaO2), low O2 content (CaO2) (Correct Answer)
- E. Low PaO2, normal O2 saturation (SaO2), normal O2 content (CaO2)
Henderson-Hasselbalch equation Explanation: ***Normal PaO2, normal O2 saturation (SaO2), low O2 content (CaO2)***
- Patients with **Crohn's disease** are prone to developing **iron deficiency anemia** due to chronic inflammation, malabsorption, and blood loss, leading to reduced hemoglobin levels.
- While PaO2 and SaO2 measure oxygen *tension* and *percentage saturation* of available hemoglobin, respectively, **O2 content (CaO2)** directly reflects the *total amount* of oxygen delivered to tissues, which is primarily dependent on hemoglobin concentration. Therefore, with anemia, CaO2 will be low despite normal PaO2 and SaO2 because there is less hemoglobin to carry oxygen.
*High PaO2, normal O2 saturation (SaO2), normal O2 content (CaO2)*
- High PaO2 would indicate **hyperoxygenation**, which is not an expected complication of Crohn's disease or its associated anemia.
- Normal O2 content is inconsistent with the presence of anemia, which significantly reduces the body's total oxygen-carrying capacity.
*Low PaO2, low O2 saturation (SaO2), low O2 content (CaO2)*
- Low PaO2 and SaO2 suggest a primary **respiratory problem** or severe hypoxemia, which is not directly linked to Crohn's disease or the typical presentation of iron deficiency anemia.
- While low O2 content is correct for anemia, the accompanying low PaO2 and SaO2 indicate a different underlying pathology for oxygen transport issues.
*Normal PaO2, normal O2 saturation (SaO2), normal O2 content (CaO2)*
- This finding would indicate **normal oxygenation** and oxygen-carrying capacity, which is contrary to the clinical scenario of a patient with Crohn's likely complicated by anemia.
- The patient's symptoms of headaches and fatigue are consistent with poor tissue oxygenation, which would not occur if all these parameters were normal.
*Low PaO2, normal O2 saturation (SaO2), normal O2 content (CaO2)*
- A low PaO2 with a normal SaO2 is physiologically unlikely unless there is a **left shift of the oxygen dissociation curve** with adequate hemoglobin, which doesn't fit the expected anemic state.
- Normal O2 content would rule out the presence of anemia as a cause for the symptoms, which is a common complication in Crohn's disease.
Henderson-Hasselbalch equation US Medical PG Question 8: A 44-year-old Caucasian male complains of carpopedal spasms, peri-oral numbness, and paresthesias of the hands and feet. His wife also mentions that he had a seizure not too long ago. His past surgical history is significant for total thyroidectomy due to papillary thyroid carcinoma. They then realized all of the symptoms occurred after the surgery. Which of the following would be present in this patient?
- A. Chvostek sign, QT prolongation, increased PTH, decreased serum calcium, decreased serum phosphate
- B. Chvostek sign, QT prolongation, decreased PTH, decreased serum calcium, increased serum phosphate (Correct Answer)
- C. Chvostek sign, QT shortening, increased PTH, increased serum calcium, increased serum phosphate
- D. Chvostek sign, QT shortening, decreased PTH, decreased serum calcium, increased serum phosphate
- E. Chvostek sign, QT prolongation, decreased PTH, increased serum calcium, decreased serum phosphate
Henderson-Hasselbalch equation Explanation: ***Chvostek sign, QT prolongation, decreased PTH, decreased serum calcium, increased serum phosphate***
- The patient's symptoms of carpopedal spasms, peri-oral numbness, paresthesias, and seizures following a **total thyroidectomy** are classic signs of **hypocalcemia** due to **hypoparathyroidism**. This would lead to a **decreased PTH** level, which in turn causes **decreased serum calcium** and **increased serum phosphate** due to impaired renal phosphate excretion.
- **Hypocalcemia** characteristically causes **QT prolongation** on EKG and can manifest as the **Chvostek sign** (facial muscle twitching when tapping the facial nerve).
*Chvostek sign, QT prolongation, increased PTH, decreased serum calcium, decreased serum phosphate*
- This option incorrectly states **increased PTH**. The symptoms are due to iatrogenic hypoparathyroidism following thyroidectomy, which results in **decreased PTH** production.
- While **decreased serum calcium** and **Chvostek sign** are consistent, the PTH and phosphate levels described here are characteristic of **pseudohypoparathyroidism** or vitamin D deficiency, not post-surgical hypoparathyroidism.
*Chvostek sign, QT shortening, increased PTH, increased serum calcium, increased serum phosphate*
- **QT shortening** is typically associated with **hypercalcemia**, not hypocalcemia, and the patient's symptoms are indicative of hypocalcemia.
- **Increased serum calcium** and **increased PTH** would point towards primary hyperparathyroidism, which is the opposite of the clinical picture presented.
*Chvostek sign, QT shortening, decreased PTH, decreased serum calcium, increased serum phosphate*
- This option incorrectly states **QT shortening**. **Hypocalcemia** is known to cause **QT prolongation**, not shortening.
- While other findings like **decreased PTH**, **decreased serum calcium**, and **increased serum phosphate** are consistent with post-thyroidectomy hypoparathyroidism, the erroneous QT interval change makes this option incorrect.
*Chvostek sign, QT prolongation, decreased PTH, increased serum calcium, decreased serum phosphate*
- This option incorrectly states **increased serum calcium**. The symptoms of carpopedal spasms and paresthesias are classic manifestations of **hypocalcemia**.
- Additionally, **decreased serum phosphate** in the setting of decreased PTH would be unusual; **hypoparathyroidism** typically leads to **hyperphosphatemia** due to reduced renal phosphate excretion.
Henderson-Hasselbalch equation US Medical PG Question 9: A 41-year-old man presents to urgent care with a 1-week history of severe diarrhea. He says that he has been having watery stools every 2-3 hours. The stools do not contain blood and do not float. On presentation, he is observed to have significant facial flushing, and laboratory tests reveal the following:
Serum:
Na+: 137 mEq/L
K+: 2.7 mEq/L
Cl-: 113 mEq/L
HCO3-: 14 mEq/L
A computed tomography scan reveals a small intra-abdominal mass. Staining of this mass would most likely reveal production of which of the following?
- A. Insulin
- B. Glucagon
- C. Vasoactive intestinal peptide (Correct Answer)
- D. Somatostatin
- E. Gastrin
Henderson-Hasselbalch equation Explanation: ***Vasoactive intestinal peptide***
- The patient's presentation with **severe watery diarrhea**, **hypokalemia**, **metabolic acidosis** (low HCO3-), and **facial flushing** in the presence of an **intra-abdominal mass** is highly indicative of a **VIPoma**.
- VIPomas are neuroendocrine tumors that secrete large amounts of **vasoactive intestinal peptide (VIP)**, which stimulates intestinal fluid secretion and inhibits absorption, leading to characteristic symptoms.
*Insulin*
- **Insulinomas** typically cause symptoms related to **hypoglycemia** (e.g., sweating, palpitations, confusion), which are not described in this patient.
- While insulinomas can be associated with an intra-abdominal mass, the patient's symptoms are inconsistent with excessive insulin production.
*Glucagon*
- **Glucagonomas** classically present with a syndrome including **necrolytic migratory erythema**, **diabetes mellitus**, and weight loss, along with diarrhea in some cases.
- The distinct skin rash and hyperglycemia are absent in this patient's presentation.
*Somatostatin*
- **Somatostatinomas** are often associated with a triad of **diabetes mellitus**, **cholelithiasis**, and **steatorrhea**, and sometimes hypochlorhydria.
- The patient's symptoms of severe watery diarrhea and flushing are not typical of somatostatin overproduction.
*Gastrin*
- **Gastrinomas** (Zollinger-Ellison syndrome) cause severe **peptic ulcer disease** due to excessive gastric acid secretion, often leading to abdominal pain and chronic diarrhea.
- While diarrhea can occur, the prominent hypokalemia, metabolic acidosis, and flushing point away from a gastrinoma as the primary diagnosis.
Henderson-Hasselbalch equation US Medical PG Question 10: A 67-year-old woman presents to the Emergency Department complaining of weakness and fatigue. She says she caught a “stomach bug” and has not been able to eat anything without vomiting for three days. Past medical history is significant for hyperlipidemia. She takes atorvastatin and a multivitamin daily, except for the last two days due to nausea. Today her heart rate is 106/min, respiratory rate is 16/min, temperature is 37.6°C (99.7°F) and blood pressure of 110/70 mm Hg. On physical examination, her oral mucosa is dry and she looks pale and uncomfortable. She is admitted for care and administered ondansetron. An intravenous infusion of normal saline is also initiated. An arterial blood gas is collected. Which of the following results is expected to be seen in this patient?
- A. pH: 7.48, pCO2: 44 mm Hg, HCO3-: 29 mEq/L (Correct Answer)
- B. pH: 7.49, pCO2: 33 mm Hg, HCO3-: 18 mEq/L
- C. pH: 7.31, pCO2: 62 mm Hg, HCO3-: 27 mEq/L
- D. pH: 7.30, pCO2: 36 mm Hg, HCO3-: 17 mEq/L
- E. pH: 7.36, pCO2: 42 mm Hg, HCO3-: 22 mEq/L
Henderson-Hasselbalch equation Explanation: ***pH: 7.48, pCO2: 44 mm Hg, HCO3-: 29 mEq/L***
- The patient's prolonged vomiting leads to a loss of **gastric acid (HCl)**, resulting in an increase in serum bicarbonate (**metabolic alkalosis**), which is reflected by the elevated pH (7.48) and HCO3- (29 mEq/L).
- The pCO2 (44 mm Hg) is slightly elevated above normal (40 mm Hg), indicating the expected **respiratory compensation** for metabolic alkalosis (hypoventilation to retain CO2 and normalize pH).
*pH: 7.49, pCO2: 33 mm Hg, HCO3-: 18 mEq/L*
- This option indicates a **respiratory alkalosis** with partial metabolic compensation, characterized by a high pH, low pCO2, and low HCO3-.
- The patient's condition of prolonged vomiting would not lead to an acidic (low HCO3-) or hyperventilatory (low pCO2) state.
*pH: 7.31, pCO2: 62 mm Hg, HCO3-: 27 mEq/L*
- This result suggests **respiratory acidosis** with partial metabolic compensation, indicated by a low pH, high pCO2, and slightly elevated HCO3-.
- Vomiting primarily causes metabolic alkalosis due to loss of acid, not respiratory acidosis.
*pH: 7.30, pCO2: 36 mm Hg, HCO3-: 17 mEq/L*
- This option points to **metabolic acidosis** with a low pH and low HCO3-, and a near-normal pCO2, indicating minimal respiratory compensation.
- While dehydration can occur, the primary acid-base disturbance from prolonged vomiting is a loss of acid, leading to alkalosis, not acidosis.
*pH: 7.36, pCO2: 42 mm Hg, HCO3-: 22 mEq/L*
- These values fall within the **normal range**, implying no significant acid-base disturbance.
- The patient's symptoms of prolonged vomiting and dehydration would inevitably lead to an acid-base imbalance.
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