Activator protein deficiencies US Medical PG Practice Questions and MCQs
Practice US Medical PG questions for Activator protein deficiencies. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
Activator protein deficiencies US Medical PG Question 1: A 5-month-old boy is brought to his pediatrician because his parents have noticed that he has very restricted joint movement. He was born at home without prenatal care, but they say that he appeared healthy at birth. Since then, they say that he doesn't seem to move very much and is hard to arouse. Physical exam reveals coarse facial structures and hepatosplenomegaly. Radiography reveals skeletal malformations, and serum tests show high plasma levels of lysosomal enzymes. The production of which of the following substances will most likely be disrupted in this patient?
- A. Glucocerebroside
- B. Mannose-6-phosphate (Correct Answer)
- C. Heparan sulfate
- D. Ceramide
Activator protein deficiencies Explanation: ***Mannose-6-phosphate***
- The patient's symptoms (restricted joint movement, coarse facial features, hepatosplenomegaly, skeletal malformations, and high plasma levels of lysosomal enzymes) are highly suggestive of **I-cell disease (mucolipidosis type II)**.
- I-cell disease is caused by a deficiency in the enzyme **N-acetylglucosaminyl-1-phosphotransferase**, which is responsible for phosphorylating mannose residues to create **mannose-6-phosphate (M6P)** tags; this tag is crucial for directing lysosomal enzymes to the lysosome. Without these tags, lysosomal enzymes are secreted extracellularly (hence high plasma levels) instead of being delivered to lysosomes, leading to accumulation of undigested substrates within lysosomes.
*Glucocerebroside*
- This is a substrate that accumulates in **Gaucher disease**, a **lysosomal storage disorder** caused by a deficiency in glucocerebrosidase.
- While Gaucher disease involves hepatosplenomegaly and skeletal issues, it does not typically present with the coarse facial features, severe joint restriction, or widespread undigested lysosomal enzymes in the plasma seen in this patient.
*Heparan sulfate*
- **Heparan sulfate** is a **glycosaminoglycan** that accumulates in certain mucopolysaccharidoses (e.g., Sanfilippo syndrome, Hurler syndrome).
- While mucopolysaccharidoses also present with coarse facial features, skeletal abnormalities, and hepatosplenomegaly, they are caused by defects in the enzymes that degrade glycosaminoglycans, not a defect in the lysosomal enzyme targeting mechanism itself as suggested by the high plasma lysosomal enzymes.
*Ceramide*
- **Ceramide** is a **lipid precursor** to sphingolipids and glycosphingolipids, which accumulate in various lysosomal storage diseases (e.g., Farber disease).
- While numerous lysosomal storage disorders involve improper ceramide metabolism or its derivatives, a primary defect in ceramide production or breakdown as the root cause for the entire clinical picture with high plasma lysosomal enzymes is less likely than the targeting defect in I-cell disease.
Activator protein deficiencies US Medical PG Question 2: A 12-year-old boy is brought to the emergency department because of acute onset abdominal pain. On arrival, he also complains of nausea and shortness of breath in addition to epigastric pain. He has previously been admitted to the hospital several times for respiratory infections with Pseudomonas species and uses a nebulizer and a chest wall oscillation vest at home. The patient's acute condition is found to be due to premature activation of an enzyme that normally interacts with the brush border. Which of the following describes the activity of this enzyme?
- A. Activates pancreatic enzyme precursors (Correct Answer)
- B. Breaks down elastin molecules
- C. Hydrolyzes phospholipids
- D. Digests triglycerides
- E. Activates phospholipase A2
Activator protein deficiencies Explanation: ***Activates pancreatic enzyme precursors***
- The patient's history of **recurrent respiratory infections with Pseudomonas** and use of a **nebulizer/chest wall oscillation vest** strongly suggests **cystic fibrosis (CF)**.
- In cystic fibrosis, **thickened secretions** can obstruct the pancreatic ducts, leading to **autodigestion of the pancreas** due to obstruction preventing the release of pancreatic enzymes. The enzyme being referred to is **trypsin**, which, when prematurely activated, activates other pancreatic enzyme precursors, leading to **pancreatitis**.
*Breaks down elastin molecules*
- This activity is characteristic of **elastase**, an enzyme produced by the pancreas. While elastase is involved in the overall digestive process and can be prematurely activated, its primary role is not the one alluded to in the clinical presentation, which points to **pancreatitis** from premature activation of the cascade.
- Damage to elastin is more classically associated with conditions like **emphysema** (due to alpha-1 antitrypsin deficiency) rather than acute abdominal pain secondary to autodigestion.
*Hydrolyzes phospholipids*
- This is the function of **phospholipase**, another pancreatic enzyme. While also capable of contributing to pancreatic autodigestion if prematurely activated, it is typically activated by **trypsin**, making trypsin the primary enzyme responsible for initiating the cascade of activation.
- **Phospholipase A2** acts on phospholipids, but the question describes an enzyme that *normally interacts with the brush border* before activation of the precursors begins.
*Digests triglycerides*
- This is the function of **pancreatic lipase**. Premature activation of lipase can contribute to the fat necrosis seen in pancreatitis.
- However, lipase, like many other pancreatic enzymes, is activated by **trypsin**, which is the initial enzyme in the cascade of activation leading to autodigestion.
*Activates phospholipase A2*
- This describes the action of **trypsinogen turning into trypsin**, which then activates other proenzymes like **prophospholipase A2**.
- While correct that trypsin activates phospholipase A2, the question asks about the primary enzyme whose *premature activation* causes the issue, which is **trypsin** itself, as it activates *multiple* pancreatic enzyme precursors, initiating a cascade.
Activator protein deficiencies US Medical PG Question 3: A 2-year-old boy is brought to the emergency department by his parents because of fever and recurrent episodes of jerky movements of his extremities for the past 6 hours. Pregnancy and delivery were uncomplicated, and development was normal until the age of 1 year. The parents report that he has had gradual loss of speech, vision, and motor skills over the past year. During this time, he has been admitted to the hospital three times because of myoclonic seizures. Physical examination shows hypertonicity of the upper and lower extremities. Fundoscopic examination shows pallor of the optic disc bilaterally. An MRI of the brain shows brain atrophy and hyperintensity of the periventricular and subcortical areas. Two days after admission, the patient dies. Histopathologic examination of the brain shows aggregation of globoid cells and loss of glial cells. The patient’s condition was most likely caused by a deficiency of which of the following enzymes?
- A. β-Galactocerebrosidase (Correct Answer)
- B. β-Glucocerebrosidase
- C. Arylsulfatase A
- D. Sphingomyelinase
- E. β-Hexosaminidase A
Activator protein deficiencies Explanation: ***β-Galactocerebrosidase***
- The clinical presentation, including the **rapid neurodegeneration** (loss of speech, vision, motor skills), **hypertonicity**, **optic disc pallor**, brain atrophy, and periventricular/subcortical hyperintensities on MRI, is highly consistent with **Krabbe disease**.
- The classic histopathologic finding of **globoid cells** (macrophages filled with undigested galactocerebroside) and **loss of glial cells** in the brain are pathognomonic for Krabbe disease, which is caused by a deficiency of **β-galactocerebrosidase**.
*β-Glucocerebrosidase*
- Deficiency of β-glucocerebrosidase causes **Gaucher disease**, which typically involves **hepatosplenomegaly**, **bone crises**, and **pancytopenia**.
- While some forms have neurological involvement, the characteristic globoid cells and rapid neurodegeneration seen here are not typical for Gaucher disease.
*Arylsulfatase A*
- Deficiency of arylsulfatase A leads to **metachromatic leukodystrophy (MLD)**, which also presents with **progressive neurological deterioration**, motor regression, and demyelination.
- However, MLD is characterized by the accumulation of **sulfatides** in white matter and detection of **metachromatic granules** in nerves and urine, not globoid cells.
*Sphingomyelinase*
- Deficiency of sphingomyelinase causes **Niemann-Pick disease**, which is characterized by **hepatosplenomegaly**, **cherry-red spots** in the macula (in type A), and foam cells in various tissues.
- The neurological symptoms and brain pathology in this child are not consistent with Niemann-Pick disease.
*β-Hexosaminidase A*
- Deficiency of β-hexosaminidase A causes **Tay-Sachs disease**, which presents with **progressive neurodegeneration**, **cherry-red spots** in the macula, and **exaggerated startle response**.
- While it causes loss of motor skills and vision, the severe demyelination with periventricular hyperintensities and globoid cells are not features of Tay-Sachs disease (which primarily involves ganglioside accumulation).
Activator protein deficiencies US Medical PG Question 4: An 18-month-old boy of Ashkenazi-Jewish descent presents with loss of developmental milestones. On ocular exam, a cherry-red macular spot is observed. No hepatomegaly is observed on physical exam. Microscopic exam shows lysosomes with onion-skin appearance.
What is the most likely underlying biochemical abnormality?
- A. Accumulation of ceramide trihexoside
- B. Accumulation of glucocerebroside
- C. Accumulation of galactocerebroside
- D. Accumulation of sphingomyelin
- E. Accumulation of GM2 ganglioside (Correct Answer)
Activator protein deficiencies Explanation: ***Accumulation of GM2 ganglioside***
- This constellation of symptoms—**loss of developmental milestones**, **cherry-red macular spot**, absence of hepatomegaly, and **lysosomes with onion-skin appearance** in an individual of **Ashkenazi-Jewish descent**—is classic for **Tay-Sachs disease**.
- **Tay-Sachs disease** is caused by a deficiency of **hexosaminidase A**, leading to the accumulation of **GM2 ganglioside** in neuronal lysosomes.
*Accumulation of ceramide trihexoside*
- This refers to **Fabry disease**, which is an **X-linked disorder** presenting in adolescence or adulthood with acroparesthesias, angiokeratomas, and renal/cardiac complications.
- While it involves a lysosomal storage, its clinical presentation and the absence of a cherry-red spot differentiate it from the case described.
*Accumulation of glucocerebroside*
- This is characteristic of **Gaucher disease**, which is caused by a deficiency in **glucocerebrosidase**.
- Key features include **hepatosplenomegaly**, bone pain, and pancytopenia, which are not consistent with the patient's presentation.
*Accumulation of galactocerebroside*
- This describes **Krabbe disease**, a **globoid cell leukodystrophy** caused by a deficiency in galactocerebrosidase.
- Krabbe disease primarily affects the **myelin sheath** in the nervous system, leading to neurological degeneration but typically does not present with a cherry-red macular spot.
*Accumulation of sphingomyelin*
- This is the hallmark of **Niemann-Pick disease**, caused by **sphingomyelinase deficiency**.
- While Niemann-Pick disease also presents with a **cherry-red macular spot** and neurodegeneration, it is classically associated with **hepatosplenomegaly**, which is explicitly stated to be absent in this patient.
Activator protein deficiencies US Medical PG Question 5: A deficiency in which of the following lysosomal enzymes is inherited in a pattern similar to a deficiency of iduronate sulfatase (Hunter syndrome)?
- A. Sphingomyelinase
- B. Glucocerebrosidase
- C. Galactocerebrosidase
- D. Alpha-L-iduronidase
- E. Alpha-galactosidase A (Correct Answer)
Activator protein deficiencies Explanation: ***Alpha-galactosidase A***
- A deficiency in **alpha-galactosidase A** causes **Fabry disease**, which, like Hunter syndrome (iduronate sulfatase deficiency), is inherited in an **X-linked recessive** pattern.
- Both conditions primarily affect males, with carrier females potentially exhibiting milder symptoms.
*Sphingomyelinase*
- A deficiency in sphingomyelinase leads to **Niemann-Pick disease types A and B**, which are inherited in an **autosomal recessive** pattern.
- This mode of inheritance differs from the X-linked pattern of Hunter syndrome.
*Glucocerebrosidase*
- A deficiency in glucocerebrosidase causes **Gaucher disease**, inherited in an **autosomal recessive** pattern.
- This is a common lysosomal storage disorder, but its inheritance pattern is distinct from X-linked disorders.
*Galactocerebrosidase*
- A deficiency in galactocerebrosidase causes **Krabbe disease (globoid cell leukodystrophy)**, which is inherited in an **autosomal recessive** pattern.
- Krabbe disease is a severe neurodegenerative disorder, but its genetic transmission is not X-linked.
*Alpha-L-iduronidase*
- A deficiency in **alpha-L-iduronidase** causes **Hurler syndrome (MPS I)**, which is inherited in an **autosomal recessive** pattern.
- While both Hunter and Hurler syndromes are mucopolysaccharidoses, their genetic inheritance patterns are different.
Activator protein deficiencies US Medical PG Question 6: A 6-month-old boy is referred to a geneticist after he is found to have persistent hypotonia and failure to thrive. He has also had episodes of what appears to be respiratory distress and has an enlarged heart on physical exam. There is a family history of childhood onset hypertrophic cardiomyopathy, so a biopsy is performed showing electron dense granules within the lysosomes. Genetic testing is performed showing a defect in glycogen processing. A deficiency in which of the following enzymes is most likely to be responsible for this patient's symptoms?
- A. Lysosomal alpha 1,4-glucosidase (Correct Answer)
- B. Branching enzyme
- C. Muscle phosphorylase
- D. Debranching enzyme
- E. Glucose-6-phosphatase
Activator protein deficiencies Explanation: ***Lysosomal alpha 1,4-glucosidase***
- The constellation of **hypotonia**, **failure to thrive**, **respiratory distress**, and **cardiomegaly** in an infant, along with **electron-dense granules in lysosomes** and a defect in **glycogen processing**, is characteristic of **Pompe disease (Type II glycogen storage disease)**.
- **Pompe disease** is caused by a deficiency of **lysosomal alpha 1,4-glucosidase** (also known as acid maltase), which is responsible for breaking down glycogen in lysosomes.
*Branching enzyme*
- A deficiency in **branching enzyme (amylo-alpha-1,4-to-alpha-1,6-transglucosidase)** causes **Andersen disease (Type IV glycogen storage disease)**, which typically presents with **hepatosplenomegaly**, **cirrhosis**, and **failure to thrive**.
- While it involves glycogenopathy, the specific features of **cardiomyopathy** and **lysosomal accumulation** are not primary to this disorder.
*Muscle phosphorylase*
- A deficiency in **muscle phosphorylase** causes **McArdle disease (Type V glycogen storage disease)**, which primarily affects **skeletal muscle**.
- Symptoms include **exercise intolerance**, **muscle cramps**, and **myoglobinuria**, typically presenting later in childhood or adolescence, and does not involve cardiomyopathy or lysosomal storage.
*Debranching enzyme*
- A deficiency in **debranching enzyme (alpha-1,6-glucosidase)** causes **Cori disease (Type III glycogen storage disease)**, which presents with **hepatomegaly**, **hypoglycemia**, and **muscle weakness**.
- While it can sometimes involve a milder form of cardiomyopathy, the significant **lysosomal involvement** and severe infantile onset with respiratory distress and profound hypotonia point away from Cori disease.
*Glucose-6-phosphatase*
- A deficiency in **glucose-6-phosphatase** causes **Von Gierke disease (Type I glycogen storage disease)**, characterized by **severe fasting hypoglycemia**, **lactic acidosis**, **hepatomegaly**, and **hyperlipidemia**.
- This condition primarily affects the liver and kidneys, and typically does not present with primary cardiomyopathy, hypotonia, or lysosomal glycogen accumulation.
Activator protein deficiencies US Medical PG Question 7: A 9-month-old girl is brought to the physician because of a 1-month history of poor feeding and irritability. She is at the 15th percentile for height and 5th percentile for weight. Examination shows hypotonia and wasting of skeletal muscles. Cardiopulmonary examination shows no abnormalities. There is hepatomegaly. Her serum glucose is 61 mg/dL, creatinine kinase is 100 U/L, and lactic acid is within the reference range. Urine ketone bodies are elevated. Which of the following enzymes is most likely deficient in this patient?
- A. Glucose-6-phosphatase
- B. Muscle phosphorylase
- C. Acid alpha-glucosidase
- D. Glycogen debrancher (Correct Answer)
- E. Glucocerebrosidase
Activator protein deficiencies Explanation: ***Glycogen debrancher***
- The patient's symptoms of **hepatomegaly**, **hypoglycemia**, **poor feeding**, **growth failure**, and **elevated urine ketones** in the presence of normal lactic acid suggest Type III glycogen storage disease (Cori disease), caused by a deficiency in **glycogen debrancher enzyme**.
- **Muscle wasting** and **hypotonia** are also consistent with Type III GSD, as the debranching enzyme is present in both liver and muscle.
*Glucose-6-phosphatase*
- Deficiency in **glucose-6-phosphatase** (Type I GSD, Von Gierke disease) also presents with **hepatomegaly** and **hypoglycemia**.
- However, Type I GSD is characterized by **lactic acidosis**, which is explicitly stated as normal in this patient, and **hyperlipidemia**, which is not mentioned.
*Muscle phosphorylase*
- Deficiency in **muscle phosphorylase** (Type V GSD, McArdle disease) primarily affects skeletal muscle, causing **exercise intolerance** and **muscle pain**.
- It does not typically present with **hypoglycemia**, **hepatomegaly**, or **growth failure** in infancy.
*Acid alpha-glucosidase*
- Deficiency in **acid alpha-glucosidase** (Type II GSD, Pompe disease) causes accumulation of glycogen in lysosomes, leading to severe **cardiomyopathy**, **hypotonia**, and **muscle weakness**.
- While hypotonia is present, the absence of **cardiomegaly** and significant **liver involvement** makes this diagnosis less likely.
*Glucocerebrosidase*
- Deficiency in **glucocerebrosidase** causes Gaucher disease, a lysosomal storage disorder, not a glycogen storage disorder.
- Symptoms include **hepatosplenomegaly**, **bone crises**, and neurological symptoms, but not **hypoglycemia** or isolated muscle wasting directly related to glycogen metabolism.
Activator protein deficiencies US Medical PG Question 8: A patient in a phase 1 trial for a novel epoxide reductase inhibitor, being studied for stroke prophylaxis, develops pain and erythema on the right thigh two days after starting the trial. This rapidly progresses to a purpuric rash with necrotic bullae within 24 hours. Lab results show a PTT of 29 seconds, PT of 28 seconds, and INR of 2.15. What is the most likely pathogenesis of this condition?
- A. Decreased plasmin activity
- B. Decreased platelet count
- C. Decreased protein C levels (Correct Answer)
- D. Increased factor VIII activity
- E. Decreased antithrombin III activity
Activator protein deficiencies Explanation: ***Decreased protein C levels***
- The clinical presentation of **pain and erythema progressing to purpuric rash with necrotic bullae** within 2-3 days of starting therapy, along with elevated PT/INR, is **pathognomonic for warfarin-induced skin necrosis**.
- This novel **epoxide reductase inhibitor** works like warfarin by inhibiting **vitamin K epoxide reductase**, which depletes all vitamin K-dependent factors.
- **Protein C and protein S** (natural anticoagulants) have **short half-lives** (6-8 hours) and drop rapidly, while procoagulant factors II, VII, IX, and X have longer half-lives (24-60 hours).
- This creates a **transient hypercoagulable state** in the first 2-3 days of therapy with **low protein C/S** but relatively preserved procoagulant factors, leading to **microvascular thrombosis** and skin necrosis.
- Most common in patients with **hereditary protein C or S deficiency** or those receiving loading doses.
*Decreased antithrombin III activity*
- Antithrombin III is **not a vitamin K-dependent factor** and is not directly affected by epoxide reductase inhibitors.
- Decreased antithrombin III would cause thrombosis but does not explain the **specific temporal relationship** and mechanism of warfarin-induced skin necrosis.
- Antithrombin III deficiency causes **venous thromboembolism**, not the characteristic cutaneous necrosis pattern.
*Decreased plasmin activity*
- Plasmin is involved in **fibrinolysis** and is not affected by vitamin K epoxide reductase inhibitors.
- Decreased plasmin activity would impair clot breakdown but does not explain the **early hypercoagulable state** specific to warfarin initiation.
- This mechanism is not relevant to warfarin-induced skin necrosis.
*Decreased platelet count*
- The lab values provided show **elevated PT/INR**, consistent with coagulation factor depletion, not thrombocytopenia.
- Thrombocytopenia causes **petechiae and mucosal bleeding**, not the large **necrotic bullae** seen here.
- Platelet count is not affected by epoxide reductase inhibitors.
*Increased factor VIII activity*
- Factor VIII is **not a vitamin K-dependent factor** and is not depleted by epoxide reductase inhibitors.
- While elevated factor VIII can contribute to hypercoagulability, it does not explain the **specific mechanism and timeline** of warfarin-induced skin necrosis.
- This is not the primary pathogenesis of this condition.
Activator protein deficiencies US Medical PG Question 9: A 24-year-old woman comes to the physician because of progressively worsening episodes of severe, crampy abdominal pain and nonbloody diarrhea for the past 3 years. Examination of the abdomen shows mild distension and generalized tenderness. There is a fistula draining stool in the perianal region. Immunohistochemistry shows dysfunction of the nucleotide oligomerization binding domain 2 (NOD2) protein. This dysfunction most likely causes overactivity of which of the following immunological proteins in this patient?
- A. Interferon-γ
- B. β-catenin
- C. IL-1β
- D. IL-10
- E. NF-κB (Correct Answer)
Activator protein deficiencies Explanation: ***NF-κB***
- **NOD2** is a pattern recognition receptor that normally detects bacterial products and regulates inflammatory responses. In **Crohn's disease**, loss-of-function **NOD2 mutations** lead to impaired bacterial sensing and clearance.
- This defective NOD2 function results in **compensatory overactivation of NF-κB** through alternative inflammatory pathways (particularly TLR signaling), causing excessive **pro-inflammatory cytokine** production.
- This **NF-κB hyperactivation** is a key driver of chronic inflammation in **Crohn's disease**, contributing to symptoms like fistulas, strictures, and transmural inflammation.
*Interferon-γ*
- **Interferon-γ** is an important pro-inflammatory cytokine in Crohn's disease and is part of the Th1-mediated immune response.
- However, its production is downstream of **NF-κB** activation and other inflammatory cascades. **NOD2 dysfunction** does not directly cause **IFN-γ** overactivity through the primary molecular pathway.
*β-catenin*
- **β-catenin** is a key component of the **Wnt signaling pathway** involved in cell adhesion, proliferation, and differentiation.
- It is not directly affected by **NOD2 dysfunction**. Dysregulation of **β-catenin** is more commonly associated with colorectal adenomas and cancer, not the inflammatory mechanisms of Crohn's disease.
*IL-1β*
- **IL-1β** is a potent pro-inflammatory cytokine that is indeed elevated in **Crohn's disease**.
- However, **IL-1β** is produced **downstream** of **NF-κB** activation. The primary molecular consequence of **NOD2 dysfunction** is the overactivity of **NF-κB**, which then drives production of various cytokines including **IL-1β**.
*IL-10*
- **IL-10** is an **anti-inflammatory cytokine** essential for maintaining intestinal immune homeostasis and suppressing excessive inflammatory responses.
- In Crohn's disease, **IL-10** signaling is often **impaired or deficient** rather than overactive. The question asks about overactivity, making this the opposite of what occurs in the disease.
Activator protein deficiencies US Medical PG Question 10: The lac operon allows E. coli to effectively utilize lactose when it is available, and not to produce unnecessary proteins. Which of the following genes is constitutively expressed and results in the repression of the lac operon?
- A. LacY
- B. LacI (Correct Answer)
- C. LacZ
- D. CAP
- E. LacA
Activator protein deficiencies Explanation: ***LacI***
- The **LacI gene** encodes the **Lac repressor protein**, which is constitutively expressed (always produced) and binds to the operator region of the lac operon.
- When bound, the **Lac repressor** blocks RNA polymerase from transcribing the structural genes (LacZ, LacY, LacA), thereby repressing the operon in the absence of lactose.
*LacY*
- The **LacY gene** encodes **lactose permease**, an enzyme responsible for transporting lactose into the bacterial cell.
- Its expression is regulated by the lac operon and is not constitutively expressed; rather, it is induced in the presence of lactose.
*LacZ*
- The **LacZ gene** encodes **beta-galactosidase**, the enzyme that breaks down lactose into glucose and galactose.
- Like LacY, its expression is part of the lac operon and is induced when lactose is available, not expressed constitutively.
*CAP*
- **CAP (Catabolite Activator Protein)** is a regulatory protein that, when bound to cAMP, activates transcription of the lac operon when glucose is absent.
- While essential for lac operon regulation, CAP is not a gene whose constitutive expression leads to repression of the operon.
*LacA*
- The **LacA gene** encodes **thiogalactoside transacetylase**, an enzyme with a less clear role in lactose metabolism but is part of the lac operon.
- Its expression is also regulated and induced along with LacZ and LacY, not constitutively expressed to repress the operon.
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