Acid-base balance during exercise US Medical PG Practice Questions and MCQs
Practice US Medical PG questions for Acid-base balance during exercise. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
Acid-base balance during exercise US Medical PG Question 1: A 24-year-old professional athlete is advised to train in the mountains to enhance his performance. After 5 months of training at an altitude of 1.5 km (5,000 feet), he is able to increase his running pace while competing at sea-level venues. Which of the following changes would produce the same effect on the oxygen-hemoglobin dissociation curve as this athlete's training did?
- A. Decreased 2,3-bisphosphoglycerate (Correct Answer)
- B. Increased carbon monoxide inhalation
- C. Decreased temperature
- D. Decreased pH
- E. Increased partial pressure of oxygen
Acid-base balance during exercise Explanation: ***Decreased 2,3-bisphosphoglycerate***
- This is **NOT** the correct physiological adaptation from altitude training, making this question conceptually flawed.
- Altitude training causes **increased erythropoietin → polycythemia → increased total hemoglobin**, which increases oxygen-carrying capacity.
- 2,3-BPG is **initially increased** at altitude (right shift) to facilitate O2 release, and remains elevated or returns to normal with acclimatization, **not decreased**.
- While decreased 2,3-BPG would cause a left shift (increased O2 affinity), this does NOT replicate altitude training adaptations.
*Increased carbon monoxide inhalation*
- Carbon monoxide binds hemoglobin with **200-250× higher affinity** than oxygen, forming carboxyhemoglobin.
- This **reduces oxygen-carrying capacity** and causes a left shift for remaining hemoglobin.
- This is harmful and does NOT replicate beneficial altitude adaptations.
*Decreased temperature*
- Decreases metabolic rate and causes a **left shift** (increased O2 affinity).
- Oxygen is held more tightly and released less readily to tissues.
- This does NOT replicate altitude training benefits.
*Decreased pH*
- Acidosis causes the **Bohr effect**: **right shift** (decreased O2 affinity).
- Facilitates O2 release to tissues during exercise.
- This is beneficial during exercise but does NOT replicate the chronic altitude adaptation of increased oxygen-carrying capacity.
*Increased partial pressure of oxygen*
- Higher PO2 increases hemoglobin saturation but does NOT shift the curve.
- This increases oxygen availability but does NOT replicate the physiological adaptation (polycythemia) from altitude training.
**Note:** This question is conceptually problematic as none of the options accurately replicate the primary altitude training adaptation (increased RBC mass/hemoglobin concentration).
Acid-base balance during exercise US Medical PG Question 2: A 24-year-old woman presents to the emergency department after she was found agitated and screaming for help in the middle of the street. She says she also has dizziness and tingling in the lips and hands. Her past medical history is relevant for general anxiety disorder, managed medically with paroxetine. At admission, her pulse is 125/min, respiratory rate is 25/min, and body temperature is 36.5°C (97.7°F). Physical examination is unremarkable. An arterial blood gas sample is taken. Which of the following results would you most likely expect to see in this patient?
- A. pH: increased, HCO3-: increased, Pco2: increased
- B. pH: decreased, HCO3-: decreased, Pco2: decreased
- C. pH: decreased, HCO3-: increased, Pco2: increased
- D. pH: increased, HCO3-: decreased, Pco2: decreased (Correct Answer)
- E. pH: normal, HCO3-: increased, Pco2: increased
Acid-base balance during exercise Explanation: ***pH: increased, HCO3-: decreased, Pco2: decreased***
- The patient's presentation with **agitation**, **dizziness**, **paresthesias** (tingling in lips and hands), and **tachypnea** (respiratory rate 25/min) is highly suggestive of **hyperventilation** due to an anxiety attack.
- **Hyperventilation** leads to excessive **CO2 expulsion**, causing a decrease in Pco2, which results in respiratory alkalosis (increased pH) and a compensatory decrease in HCO3-.
*pH: increased, HCO3-: increased, Pco2: increased*
- An **increased pH** coupled with **increased HCO3-** and **increased Pco2** would suggest a **metabolic alkalosis with respiratory compensation**, which is not consistent with the patient's acute hyperventilation.
- While pH is increased, the other values contradict the primary respiratory cause suggested by the symptoms.
*pH: decreased, HCO3-: decreased, Pco2: decreased*
- This profile describes **metabolic acidosis with respiratory compensation**, which would typically present with **Kussmaul breathing** and other signs of acidosis, not acute hyperventilation and agitation.
- Symptoms such as dizziness and tingling are associated with alkalosis, not acidosis.
*pH: decreased, HCO3-: increased, Pco2: increased*
- This pattern is characteristic of **respiratory acidosis with metabolic compensation**, often seen in conditions like **COPD exacerbation** or **opioid overdose** with hypoventilation.
- The patient's rapid breathing and clinical picture are not consistent with respiratory acidosis.
*pH: normal, HCO3-: increased, Pco2: increased*
- A **normal pH** with **increased HCO3-** and **increased Pco2** would indicate a **compensated metabolic alkalosis**.
- Her acute symptoms point to an uncompensated or acutely compensated respiratory disorder, not a compensated metabolic issue.
Acid-base balance during exercise US Medical PG Question 3: A 52-year-old man undergoes an exercise stress test for a 1-week history of squeezing substernal chest pain that is aggravated by exercise and relieved by rest. During the test, there is a substantial increase in the breakdown of glycogen in the muscle cells. Which of the following changes best explains this intracellular finding?
- A. Activation of phosphorylase kinase (Correct Answer)
- B. Decrease in protein kinase A
- C. Inactivation of glycogen synthase kinase
- D. Activation of protein phosphatase
- E. Increase in glucose-6-phosphate
Acid-base balance during exercise Explanation: ***Activation of phosphorylase kinase***
- Exercise, particularly in the context of **ischemic heart disease** suggested by the patient's symptoms, triggers a rapid need for energy, leading to **glycogenolysis**.
- **Phosphorylase kinase** is the key enzyme that activates **glycogen phosphorylase**, the rate-limiting step in glycogen breakdown, to release glucose-1-phosphate from glycogen stores.
*Decrease in protein kinase A*
- **Protein kinase A (PKA)** is typically activated during exercise via **epinephrine** signaling, which in turn *activates* phosphorylase kinase and *inhibits* glycogen synthase.
- A decrease in PKA activity would lead to *reduced* glycogen breakdown, which contradicts the described increase in glycogen breakdown.
*Inactivation of glycogen synthase kinase*
- **Glycogen synthase kinase (GSK3)** phosphorylates and inactivates **glycogen synthase**, thereby *inhibiting* glycogen synthesis.
- If GSK3 were inactivated, glycogen synthesis would be *promoted*, rather than glycogen breakdown, further contradicting the clinical scenario.
*Activation of protein phosphatase*
- **Protein phosphatases** generally remove phosphate groups, which would *deactivate* glycogen phosphorylase and *activate* glycogen synthase.
- This action would promote glycogen synthesis and inhibit glycogen breakdown, which is the opposite of the observed physiological response during exercise.
*Increase in glucose-6-phosphate*
- While **glucose-6-phosphate** is an intermediate in glycogen metabolism, an increase in its concentration would primarily signal abundant glucose and tend to *inhibit* glycogen phosphorylase and *activate* glycogen synthase.
- This effect would favor glycogen synthesis and inhibit its breakdown, making it an unlikely explanation for increased glycogen breakdown during exercise.
Acid-base balance during exercise US Medical PG Question 4: An investigator is studying muscle tissue in high-performance athletes. He obtains blood samples from athletes before and after a workout session consisting of short, fast sprints. Which of the following findings is most likely upon evaluation of blood obtained after the workout session?
- A. Decreased concentration of NADH
- B. Increased concentration of H+ (Correct Answer)
- C. Decreased concentration of lactate
- D. Increased concentration of insulin
- E. Increased concentration of ATP
Acid-base balance during exercise Explanation: ***Increased concentration of H+***
- During **anaerobic metabolism** in high-intensity exercise like sprints, pyruvate is converted to **lactate** by **lactate dehydrogenase** to regenerate NAD+. This process produces H+, leading to a decrease in pH and an increase in H+ concentration in the blood.
- The accumulation of **hydrogen ions (H+)** contributes to metabolic acidosis, muscle fatigue, and the burning sensation experienced during intense exertion.
- Blood gas analysis would show **decreased pH** and **increased H+ concentration**.
*Decreased concentration of NADH*
- NADH is primarily an **intracellular metabolite** and is not typically measured in blood samples as it does not circulate freely in significant concentrations.
- Within muscle cells during anaerobic glycolysis, NADH is consumed by lactate dehydrogenase to convert pyruvate to lactate, regenerating NAD+ for continued glycolysis.
- This option is not a realistic blood finding from a clinical laboratory perspective.
*Decreased concentration of lactate*
- **High-intensity sprints** primarily rely on **anaerobic metabolism**, which rapidly produces **lactate** from pyruvate.
- Therefore, the concentration of lactate in the blood would significantly **increase** after such a workout, not decrease.
- Elevated blood lactate is a hallmark finding after intense anaerobic exercise.
*Increased concentration of insulin*
- **Insulin** levels typically **decrease** during exercise, especially high-intensity exercise, due to **sympathetic nervous system activation** and the body's need to mobilize glucose from liver glycogen and fatty acids.
- Exercise promotes glucose uptake through **insulin-independent mechanisms** (GLUT4 translocation via AMP-activated protein kinase).
- Increased insulin would be counterproductive during intense exercise when glucose mobilization is needed.
*Increased concentration of ATP*
- ATP does not circulate in blood in measurable concentrations as a typical laboratory finding.
- Within muscle cells, ATP is rapidly **consumed** during intense exercise to fuel muscle contraction.
- While cells work to maintain ATP levels through anaerobic glycolysis and the creatine phosphate system, net ATP does not accumulate in the blood.
Acid-base balance during exercise US Medical PG Question 5: During heavy exercise, what is the primary mechanism for maintaining arterial pH despite increased lactic acid production?
- A. Increased bicarbonate reabsorption
- B. Phosphate buffering
- C. Increased hydrogen secretion
- D. Hyperventilation (Correct Answer)
Acid-base balance during exercise Explanation: ***Hyperventilation***
- **Hyperventilation** during heavy exercise increases the expulsion of **carbon dioxide (CO2)**, shifting the **bicarbonate buffer system** equilibrium to the left.
- This reduction in **CO2** effectively removes **hydrogen ions (H+)**, thereby helping to maintain **arterial pH** despite rising **lactic acid** levels.
*Increased bicarbonate reabsorption*
- While the kidneys adapt by increasing **bicarbonate reabsorption** to compensate for acidosis, this is a **slower renal mechanism** for pH regulation, taking hours to days, rather than an immediate response during acute exercise.
- The rapid pH regulation during exercise primarily relies on respiratory and chemical buffer systems, not renal function.
*Phosphate buffering*
- The **phosphate buffer system** is indeed important for intracellular and renal tubular fluid buffering.
- However, its buffering capacity in the extracellular fluid and plasma is relatively limited compared to the **bicarbonate system** due to its lower concentration.
*Increased hydrogen secretion*
- **Increased hydrogen secretion** by the renal tubules is a long-term mechanism for compensating for acidosis, which helps excrete excess **acid** and regenerate **bicarbonate**.
- This is a slow, renal regulatory process and not the primary rapid mechanism for maintaining pH during the immediate demands of heavy exercise.
Acid-base balance during exercise US Medical PG Question 6: 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)]
Acid-base balance during exercise 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]**.
Acid-base balance during exercise US Medical PG Question 7: A 70-year-old woman is brought to the emergency department due to worsening lethargy. She lives with her husband who says she has had severe diarrhea for the past few days. Examination shows a blood pressure of 85/60 mm Hg, pulse of 100/min, and temperature of 37.8°C (100.0°F). The patient is stuporous, while her skin appears dry and lacks turgor. Laboratory tests reveal:
Serum electrolytes
Sodium 144 mEq/L
Potassium 3.5 mEq/L
Chloride 115 mEq/L
Bicarbonate 19 mEq/L
Serum pH 7.3
PaO2 80 mm Hg
Pco2 38 mm Hg
This patient has which of the following acid-base disturbances?
- A. Chronic respiratory acidosis
- B. Anion gap metabolic acidosis with respiratory compensation
- C. Anion gap metabolic acidosis
- D. Non-anion gap metabolic acidosis with respiratory compensation (Correct Answer)
- E. Non-anion gap metabolic acidosis
Acid-base balance during exercise Explanation: ***Non-anion gap metabolic acidosis with respiratory compensation***
- This patient has significant **diarrhea**, which causes a loss of **bicarbonate** from the gastrointestinal tract, leading to a **non-anion gap metabolic acidosis**.
- The **serum pH of 7.3** confirms acidosis, and the **Pco2 of 38 mm Hg** (which is slightly below the normal range, considering the acidosis) indicates effective **respiratory compensation** for the metabolic disturbance. Calculating the **anion gap** = Na - (Cl + HCO3) = 144 - (115 + 19) = **10 mEq/L** (normal range 8-12 mEq/L), which is within normal limits.
*Chronic respiratory acidosis*
- This would involve an elevated **Pco2** and a compensatory increase in **bicarbonate**, neither of which are observed in this patient.
- The patient's primary problem is loss of bicarbonate due to diarrhea, not impaired CO2 excretion.
*Anion gap metabolic acidosis with respiratory compensation*
- An **anion gap metabolic acidosis** would show an elevated anion gap (>12 mEq/L), which is not present here (anion gap is 10 mEq/L).
- While respiratory compensation is occurring, the underlying acidosis is **non-anion gap**.
*Anion gap metabolic acidosis*
- This diagnosis requires an **elevated anion gap**, which is calculated as Na - (Cl + HCO3) = 144 - (115 + 19) = **10 mEq/L**.
- Since the anion gap is within the normal range, an anion gap metabolic acidosis is excluded.
*Non-anion gap metabolic acidosis*
- While the patient does have a **non-anion gap metabolic acidosis** due to bicarbonate loss from diarrhea, this option doesn't account for the **respiratory compensation** indicated by the Pco2.
- The slightly reduced Pco2 demonstrates the body's attempt to normalize pH, making "with respiratory compensation" a more complete description.
Acid-base balance during exercise US Medical PG Question 8: A 71-year-old woman comes to the physician because of an 8-month history of fatigue. Laboratory studies show a hemoglobin concentration of 13.3 g/dL, a serum creatinine concentration of 0.9 mg/dL, and a serum alkaline phosphatase concentration of 100 U/L. Laboratory evaluation of which of the following parameters would be most helpful in determining the cause of this patient's symptoms?
- A. Cancer antigen 27-29
- B. Calcitriol
- C. Lactate dehydrogenase
- D. Ferritin (Correct Answer)
- E. Gamma-glutamyl transpeptidase
Acid-base balance during exercise Explanation: ***Correct: Ferritin***
- The patient's fatigue associated with a normal hemoglobin and creatinine suggests a subtle cause for fatigue, and **iron deficiency without anemia** (iron deficiency anemia can cause decrease in hemoglobin levels) is a common cause, which would be identified by low ferritin levels.
- Ferritin is a **storage protein for iron**, and its levels accurately reflect the body's iron stores and are the most sensitive indicator for iron deficiency.
*Incorrect: Cancer antigen 27-29*
- This marker is primarily used to monitor **breast cancer recurrence** or progression, not for initial diagnosis or as a general screen for fatigue.
- There is no clinical indication in the patient's presentation that suggests breast cancer, making this an unlikely and unhelpful test.
*Incorrect: Calcitriol*
- Calcitriol is the active form of vitamin D, primarily involved in **calcium and phosphorus metabolism** and bone health.
- While vitamin D deficiency can cause fatigue, the patient's normal alkaline phosphatase (ALP is elevated in vitamin D deficiency due to secondary hyperparathyroidism) and absence of other related symptoms make calcitriol assessment less likely to be the most helpful first step.
*Incorrect: Lactate dehydrogenase*
- **LDH is a general marker of tissue damage and cell turnover**, elevated in conditions like hemolysis, malignancy, or liver disease.
- It is a non-specific marker that would not pinpoint the cause of fatigue in this patient with otherwise normal baseline labs.
*Incorrect: Gamma-glutamyl transpeptidase*
- GGT is an enzyme primarily used to assess **liver function and bile duct obstruction**, often in conjunction with alkaline phosphatase.
- Given the patient's normal alkaline phosphatase and no other signs of liver disease, GGT would not be a helpful test for fatigue in this context.
Acid-base balance during exercise US Medical PG Question 9: A 32-year-old woman is admitted to the emergency department for 36 hours of intense left-sided back pain that extends into her left groin. She reports that the pain started a day after a charitable 5 km (3.1 mi) marathon. The past medical history is relevant for multiple complaints of eye dryness and dry mouth. Physical examination is unremarkable, except for intense left-sided costovertebral pain. The results from laboratory tests are shown.
Laboratory test Result
Serum Na+ 137
Serum Cl- 110
Serum K+ 3.0
Serum creatinine (SCr) 0.82
Arterial blood gas Result
pH 7.28
pO2 98 mm Hg
pCO2 28.5 mm Hg
SaO2% 98%
HCO3- 15 mm Hg
Which of the following explains this patient’s condition?
- A. Carbonic acid accumulation
- B. Decreased bicarbonate renal absorption
- C. Decreased renal excretion of hydrogen ions (H+) (Correct Answer)
- D. Decreased synthesis of ammonia (NH3)
- E. Decreased excretion of nonvolatile acids
Acid-base balance during exercise Explanation: ***Decreased renal excretion of hydrogen ions (H+)***
- The patient presents with **metabolic acidosis** (pH 7.28, HCO3- 15 mEq/L) with **respiratory compensation** (pCO2 28.5 mm Hg). The anion gap is **normal** (Na+ - (Cl- + HCO3-) = 137 - (110 + 15) = **12 mEq/L**), indicating a **non-anion gap metabolic acidosis**.
- The history of **dry eyes and dry mouth** strongly suggests **Sjögren syndrome**, which is commonly associated with **Type 1 (distal) renal tubular acidosis**.
- In **Type 1 RTA**, the distal tubule alpha-intercalated cells cannot adequately secrete H+ ions, leading to metabolic acidosis with **inability to acidify urine** (urine pH > 5.5). Associated findings include **hypokalemia** (K+ 3.0), **nephrolithiasis** (calcium phosphate stones due to alkaline urine), and hypercalciuria.
- The left-sided flank pain radiating to the groin is consistent with **nephrolithiasis**, a common complication of Type 1 RTA.
*Carbonic acid accumulation*
- **Carbonic acid accumulation** indicates **respiratory acidosis** with elevated pCO2, which is not present here.
- The patient has a **low pCO2 (28.5 mm Hg)**, representing appropriate **respiratory compensation** for the primary metabolic acidosis.
*Decreased bicarbonate renal absorption*
- **Decreased bicarbonate renal absorption** characterizes **Type 2 (proximal) RTA**.
- While Type 2 RTA also causes non-anion gap metabolic acidosis, it is **not typically associated with Sjögren syndrome** and would present with different features (glycosuria, aminoaciduria, phosphaturia as part of Fanconi syndrome).
- Type 2 RTA can acidify urine to pH < 5.5 when serum HCO3- is low, unlike Type 1 RTA.
*Decreased synthesis of ammonia (NH3)*
- **Decreased ammonia synthesis** is characteristic of **Type 4 RTA** or severe chronic kidney disease.
- Type 4 RTA presents with **hyperkalemia** (due to hypoaldosteronism), not the hypokalemia seen in this patient.
- The normal serum creatinine (0.82 mg/dL) rules out significant renal failure.
*Decreased excretion of nonvolatile acids*
- **Decreased excretion of nonvolatile acids** would cause **elevated anion gap metabolic acidosis** (e.g., lactic acidosis, ketoacidosis, or advanced renal failure with accumulation of organic acids).
- This patient has a **normal anion gap (12 mEq/L)** and **normal renal function** (creatinine 0.82 mg/dL), making this mechanism unlikely.
- The clinical context of Sjögren syndrome with dry eyes/mouth points specifically to distal RTA.
Acid-base balance during exercise US Medical PG Question 10: 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
Acid-base balance during exercise 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.
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