A pharmaceutical company is developing a novel therapy for Type I glycogen storage disease using adeno-associated virus (AAV) vectors to deliver the glucose-6-phosphatase gene. Phase I trials show successful hepatic gene transfer with 30% of normal enzyme activity, resulting in improved fasting glucose and reduced hepatomegaly. However, some patients develop immune responses to the viral vector and lose therapeutic benefit after 6 months. Synthesize the therapeutic approach and evaluate strategies to optimize long-term efficacy.
Q2
A 25-year-old woman with Type I glycogen storage disease has been managed with frequent feedings and nocturnal gastric drip feeding since childhood. She now presents for preconception counseling. Her current labs show: fasting glucose 65 mg/dL, lactate 4 mmol/L, uric acid 8 mg/dL, triglycerides 400 mg/dL, and ALT 150 U/L. Renal ultrasound shows bilateral adenomas. She desires pregnancy but is concerned about risks. Synthesize the multisystem complications and evaluate the pregnancy management approach.
Q3
A neonate presents with severe hypotonia, feeding difficulties, and respiratory distress requiring mechanical ventilation. Echocardiography reveals severe hypertrophic cardiomyopathy with left ventricular outflow obstruction. Genetic testing confirms Pompe disease, and enzyme replacement therapy (ERT) with recombinant alpha-glucosidase is initiated. At 6 months, the infant shows improved motor function and reduced cardiomegaly, but cognitive development remains delayed. The parents question the differential response. Evaluate the pathophysiological basis for this treatment response pattern.
Q4
A 15-year-old athlete undergoes genetic testing after experiencing exercise-induced myoglobinuria. Testing reveals a homozygous mutation in the PYGM gene. His younger brother, age 12, is found to be heterozygous for the same mutation and is asymptomatic. The parents request guidance on the younger brother's athletic participation. Analyze the genotype-phenotype correlation and evaluate the appropriate counseling.
Q5
A 5-year-old boy with known glycogen storage disease presents to the emergency department with altered mental status. He missed his nighttime cornstarch feeding due to viral gastroenteritis with vomiting. Labs show: glucose 35 mg/dL, lactate 8 mmol/L (elevated), uric acid 12 mg/dL (elevated), triglycerides 600 mg/dL (elevated), and mild metabolic acidosis. His abdomen is distended with massive hepatomegaly. Analyze the metabolic derangements and determine the priority management.
Q6
A 2-year-old boy presents with progressive hepatosplenomegaly and failure to thrive. Liver biopsy shows accumulation of abnormal glycogen with long outer branches and fewer branch points. The child develops portal hypertension and ascites. Laboratory evaluation reveals hypoglycemia is not prominent, but liver function tests are markedly abnormal with elevated transaminases and bilirubin. Analyze these findings to determine the most likely diagnosis and underlying pathophysiology.
Q7
A 3-year-old boy presents with hepatomegaly, growth retardation, and fasting hypoglycemia that is less severe than his sibling who has Von Gierke disease. Laboratory studies show elevated transaminases and mild hyperlipidemia, but normal lactate and uric acid levels. Unlike his sibling, he responds to glucagon administration with increased blood glucose when fed, but not after prolonged fasting. Apply these findings to identify the enzymatic defect.
Q8
A 4-year-old girl presents with progressive muscle weakness, hypotonia, and cardiomegaly. Chest X-ray shows massive cardiomegaly with pulmonary congestion. Echocardiography reveals severe left ventricular hypertrophy with outflow obstruction. Electromyography shows myopathic changes, and serum creatine kinase is markedly elevated. Muscle biopsy shows vacuoles filled with glycogen and increased lysosomal acid phosphatase activity. Apply this information to determine the deficient enzyme.
Q9
A 20-year-old man presents with muscle cramps and myoglobinuria after moderate exercise. He reports that he gets a 'second wind' if he rests briefly during exercise and then continues. Ischemic forearm exercise test shows no rise in venous lactate. Muscle biopsy reveals periodic acid-Schiff (PAS)-positive material. Apply these findings to identify the underlying enzyme deficiency.
Q10
A 6-month-old infant presents with severe hypoglycemia, hepatomegaly, and lactic acidosis. Physical examination reveals a doll-like face with fat cheeks and protuberant abdomen. Laboratory studies show elevated serum lactate, uric acid, and triglycerides. A liver biopsy demonstrates excessive glycogen accumulation. Glucagon administration fails to increase blood glucose levels. Apply your knowledge to determine the most likely enzymatic defect.
Glycogen storage diseases US Medical PG Practice Questions and MCQs
Question 1: A pharmaceutical company is developing a novel therapy for Type I glycogen storage disease using adeno-associated virus (AAV) vectors to deliver the glucose-6-phosphatase gene. Phase I trials show successful hepatic gene transfer with 30% of normal enzyme activity, resulting in improved fasting glucose and reduced hepatomegaly. However, some patients develop immune responses to the viral vector and lose therapeutic benefit after 6 months. Synthesize the therapeutic approach and evaluate strategies to optimize long-term efficacy.
A. Increase initial viral vector dose to achieve higher enzyme expression
B. Switch to ex vivo gene therapy using autologous hepatocytes
C. Combine gene therapy with traditional dietary management only
D. Develop tolerance induction protocol before gene therapy administration (Correct Answer)
E. Add systemic immunosuppression at therapy initiation to prevent immune response
Explanation: ***Develop tolerance induction protocol before gene therapy administration***
- Developing a **tolerance induction protocol** (e.g., using regulatory T-cell induction or B-cell depletion) addresses the root cause of treatment failure by preventing the **neutralizing antibody** and **cytotoxic T-cell** responses against the AAV vector.
- This facilitates long-term **transgene expression** and may allow for safer **vector re-administration** if enzyme levels decline as hepatocytes proliferate over time.
*Increase initial viral vector dose to achieve higher enzyme expression*
- Increasing the **viral load** often exacerbates the **innate and adaptive immune responses**, leading to more severe **capsid-specific T-cell** activation and acute hepatotoxicity.
- Higher doses do not solve the problem of **immunological memory**, which is the primary reason patients lose therapeutic benefits after 6 months.
*Add systemic immunosuppression at therapy initiation to prevent immune response*
- While transient immunosuppression (like steroids) is common, it may not be sufficient for **durable tolerance** if the immune system recognizes the **transgene product** as foreign later on.
- Long-term systemic immunosuppression carries significant risks of **opportunistic infections** and does not actively reprogram the immune system to accept the **G6Pase enzyme**.
*Switch to ex vivo gene therapy using autologous hepatocytes*
- **Ex vivo gene therapy** involving hepatocyte harvest, transduction, and re-infusion is technically difficult and often results in poor **engraftment** compared to in vivo AAV delivery.
- This method does not necessarily bypass the immune response against the **intracellular enzyme** if the patient's immune system lacks **central tolerance** to the G6Pase protein.
*Combine gene therapy with traditional dietary management only*
- **Dietary management** via cornstarch or frequent feeding only addresses the **symptoms** of hypoglycemia and does not stop the underlying **metabolic dysregulation** or vector loss.
- This strategy fails to optimize the **efficacy** of the gene therapy itself, which is the specific goal of the intervention described in the prompt.
Question 2: A 25-year-old woman with Type I glycogen storage disease has been managed with frequent feedings and nocturnal gastric drip feeding since childhood. She now presents for preconception counseling. Her current labs show: fasting glucose 65 mg/dL, lactate 4 mmol/L, uric acid 8 mg/dL, triglycerides 400 mg/dL, and ALT 150 U/L. Renal ultrasound shows bilateral adenomas. She desires pregnancy but is concerned about risks. Synthesize the multisystem complications and evaluate the pregnancy management approach.
A. Pregnancy is contraindicated due to metabolic instability and should be avoided
B. Recommend liver transplantation before attempting pregnancy
C. Use insulin pump therapy to maintain glucose stability during pregnancy
D. Optimize metabolic control pre-conception, then manage with multidisciplinary team during pregnancy (Correct Answer)
E. Proceed with pregnancy using continuous glucose monitoring and frequent small meals
Explanation: ***Optimize metabolic control pre-conception, then manage with multidisciplinary team during pregnancy***
- Preconception optimization focuses on minimizing **hyperlactatemia**, **hyperuricemia**, and **hypertriglyceridemia** to reduce risks of **preeclampsia** and metabolic crises.
- A **multidisciplinary team** (high-risk OB, metabolic specialist, nephrologist) is essential to monitor for **hepatic adenoma expansion** and manage the increased glucose requirement of pregnancy.
*Pregnancy is contraindicated due to metabolic instability and should be avoided*
- Pregnancy is not absolute contraindication in **GSD Type I**, as many women have successful outcomes with strict **glycemic management** and metabolic monitoring.
- While high-risk, a contraindication ignores advances in **nocturnal cornstarch therapy** and monitoring that allow for controlled metabolic states.
*Proceed with pregnancy using continuous glucose monitoring and frequent small meals*
- This approach is incomplete because it fails to address the risk of **hepatic adenoma hemorrhage** driven by increased **estrogen levels** during pregnancy.
- Managing only the glucose misses critical monitoring for **renal dysfunction** and the potential need for **cesarean delivery** due to maternal pelvic complications.
*Recommend liver transplantation before attempting pregnancy*
- **Liver transplantation** is reserved for patients with uncontrollable metabolic derangements or **hepatocellular carcinoma**, not as a routine prerequisite for pregnancy.
- Post-transplant pregnancy involves significant risks from **immunosuppressive drugs** and possible **allograft rejection**, making it a more complex alternative.
*Use insulin pump therapy to maintain glucose stability during pregnancy*
- **Insulin pump therapy** is contraindicated as it would worsen the **hypoglycemia** inherent to GSD Ia; patients lack the ability to perform **gluconeogenesis** and glycogenolysis.
- The primary treatment is exogenous **glucose/cornstarch supplementation**, as the underlying pathology is a deficiency in **glucose-6-phosphatase**, not insulin deficiency.
Question 3: A neonate presents with severe hypotonia, feeding difficulties, and respiratory distress requiring mechanical ventilation. Echocardiography reveals severe hypertrophic cardiomyopathy with left ventricular outflow obstruction. Genetic testing confirms Pompe disease, and enzyme replacement therapy (ERT) with recombinant alpha-glucosidase is initiated. At 6 months, the infant shows improved motor function and reduced cardiomegaly, but cognitive development remains delayed. The parents question the differential response. Evaluate the pathophysiological basis for this treatment response pattern.
A. ERT cannot cross the blood-brain barrier, limiting CNS glycogen clearance
B. Cognitive delay is unrelated to Pompe disease and represents a separate condition
C. Insufficient ERT dosing for CNS penetration compared to peripheral tissues
D. Cardiac and skeletal muscle have higher lysosomal enzyme uptake than neurons (Correct Answer)
E. Irreversible neuronal damage occurred before treatment initiation
Explanation: ***Cardiac and skeletal muscle have higher lysosomal enzyme uptake than neurons***
- Recombinant **human alpha-glucosidase** effectively reduces glycogen in **cardiac and skeletal muscles** because these tissues express high levels of **mannose-6-phosphate receptors**, facilitating enzyme endocytosis.
- However, this enzyme does not cross the **blood-brain barrier (BBB)**, meaning **lysosomal glycogen accumulation** continues in the CNS, leading to persistent **cognitive and neurodevelopmental delays**.
*ERT cannot cross the blood-brain barrier, limiting CNS glycogen clearance*
- While it is true that systemic **ERT** does not penetrate the **BBB**, the standard explanation for therapeutic differential focuses on the enzyme's specific **targeting mechanism** to peripheral tissues.
- The primary clinical challenge is that current therapies are optimized for **M6P receptor-mediated uptake** in muscles, leaving the **central nervous system** as a sanctuary site for glycogen deposition.
*Irreversible neuronal damage occurred before treatment initiation*
- Although early treatment is vital, the persistence of **cognitive delay** is primarily due to the ongoing lack of enzyme delivery to the **brain** post-treatment, not just pre-existing damage.
- Studies show that even with very early initiation, **central neurological pathology** progresses because the recombinant enzyme cannot reach the **neuronal lysosomes**.
*Cognitive delay is unrelated to Pompe disease and represents a separate condition*
- **Infantile-onset Pompe disease** is multi-systemic and is well-documented to cause **CNS involvement**, including glycogen storage in **motoneurons** and glial cells.
- Attributing cognitive deficits to a separate condition ignores the known **pathophysiology** of GSD II in long-term survivors treated with systemic **ERT**.
*Insufficient ERT dosing for CNS penetration compared to peripheral tissues*
- The failure of CNS response is not a matter of **dosage**; the high molecular weight of **rhGAA** makes it structurally incapable of crossing the **intact blood-brain barrier** regardless of the dose.
- Increasing the systemic dose would only enhance **peripheral uptake** and risk immunogenic reactions without achieving therapeutic levels in the **cerebrospinal fluid**.
Question 4: A 15-year-old athlete undergoes genetic testing after experiencing exercise-induced myoglobinuria. Testing reveals a homozygous mutation in the PYGM gene. His younger brother, age 12, is found to be heterozygous for the same mutation and is asymptomatic. The parents request guidance on the younger brother's athletic participation. Analyze the genotype-phenotype correlation and evaluate the appropriate counseling.
A. Restrict all athletic activities as he will eventually develop symptoms
B. Perform muscle biopsy to confirm carrier status before clearance
C. Advise complete avoidance of anaerobic exercise only
D. Recommend prophylactic creatine supplementation before exercise
E. Allow normal athletic participation with avoidance of isometric exercises (Correct Answer)
Explanation: ***Allow normal athletic participation with avoidance of isometric exercises***
- The younger brother is a **heterozygous carrier** of an **autosomal recessive** condition (McArdle disease, GSD Type V); carriers typically possess sufficient **myophosphorylase** activity to remain asymptomatic.
- While carriers generally do not require restrictions, avoiding extreme **isometric exercises** or heavy weightlifting is a conservative safety measure to prevent rare instances of muscle strain in those with slightly reduced enzyme kinetics.
*Restrict all athletic activities as he will eventually develop symptoms*
- Since McArdle disease follows an **autosomal recessive inheritance pattern**, a single mutation (heterozygosity) does not lead to the progressive clinical manifestations seen in homozygotes.
- Restricting activity is medically unnecessary and can negatively impact the **cardiovascular health** and psychological well-being of a healthy carrier.
*Recommend prophylactic creatine supplementation before exercise*
- **Creatine supplementation** is sometimes considered for symptomatic homozygous patients to improve muscle performance but has no proven benefit for **asymptomatic carriers**.
- Carriers have normal energy metabolism during exercise, making pharmacological or nutritional prophylaxis redundant.
*Perform muscle biopsy to confirm carrier status before clearance*
- **Genetic testing** has already confirmed the heterozygous status; a **muscle biopsy** is invasive and unnecessary to determine athletic clearance in a carrier.
- Histology in a carrier would likely show nearly normal **glycogen levels** and sufficient enzyme activity, adding no clinical value to the management plan.
*Advise complete avoidance of anaerobic exercise only*
- Unlike affected homozygotes who struggle with **anaerobic glycolysis**, carriers can efficiently utilize muscle glycogen and do not face the same risk of **rhabdomyolysis** during bursts of activity.
- Total avoidance of anaerobic exercise is overly restrictive and not supported by the pathophysiology of the **heterozygous state**.
Question 5: A 5-year-old boy with known glycogen storage disease presents to the emergency department with altered mental status. He missed his nighttime cornstarch feeding due to viral gastroenteritis with vomiting. Labs show: glucose 35 mg/dL, lactate 8 mmol/L (elevated), uric acid 12 mg/dL (elevated), triglycerides 600 mg/dL (elevated), and mild metabolic acidosis. His abdomen is distended with massive hepatomegaly. Analyze the metabolic derangements and determine the priority management.
A. Immediate glucagon administration to mobilize hepatic glycogen stores
B. Allopurinol administration to address hyperuricemia
C. Insulin therapy to address metabolic acidosis
D. High-protein feeding to stimulate gluconeogenesis
E. Intravenous dextrose infusion with frequent glucose monitoring (Correct Answer)
Explanation: ***Intravenous dextrose infusion with frequent glucose monitoring***
- Immediate correction of severe **hypoglycemia** is the priority to manage **altered mental status** and prevent neurological damage in **Von Gierke disease** (GSD I).
- Restoring blood glucose levels suppresses the excessive **glycolysis** and **lipolysis** that drive the secondary **lactic acidosis**, **hypertriglyceridemia**, and **hyperuricemia** seen in GSD I.
*Immediate glucagon administration to mobilize hepatic glycogen stores*
- **Glucagon** is ineffective because the deficiency in **glucose-6-phosphatase** prevents the liver from releasing free glucose into the bloodstream regardless of hormonal stimuli.
- Administering glucagon may exacerbate **lactic acidosis** by promoting the breakdown of glycogen into **lactate** precursors that cannot be converted to glucose.
*High-protein feeding to stimulate gluconeogenesis*
- **Gluconeogenesis** is fundamentally impaired in GSD Type I because the final common step (conversion of glucose-6-phosphate to glucose) is blocked.
- While high-protein diets are useful in GSD Type III, they provide no benefit for blood glucose maintenance in **Von Gierke disease** and do not address the acute emergency.
*Allopurinol administration to address hyperuricemia*
- **Hyperuricemia** in GSD I is a secondary metabolic derangement caused by decreased renal clearance of urate and increased **purine degradation**; it is not the acute life-threatening issue.
- Management of the underlying **hypoglycemia** and lactic acidosis will naturally lower uric acid levels over time; **allopurinol** is used for long-term maintenance, not acute stabilization.
*Insulin therapy to address metabolic acidosis*
- **Insulin** administration would be dangerous as it would severely worsen the existing **hypoglycemia**, potentially leading to seizures or death.
- The **metabolic acidosis** is caused by lactate, not ketones from insulin deficiency, therefore the treatment is to provide glucose to stop the overproduction of **lactic acid**.
Question 6: A 2-year-old boy presents with progressive hepatosplenomegaly and failure to thrive. Liver biopsy shows accumulation of abnormal glycogen with long outer branches and fewer branch points. The child develops portal hypertension and ascites. Laboratory evaluation reveals hypoglycemia is not prominent, but liver function tests are markedly abnormal with elevated transaminases and bilirubin. Analyze these findings to determine the most likely diagnosis and underlying pathophysiology.
A. Type I glycogen storage disease with hepatic fibrosis
B. Type VI glycogen storage disease with hepatomegaly
C. Type IX glycogen storage disease with liver dysfunction
D. Type IV glycogen storage disease with amylopectin-like glycogen (Correct Answer)
E. Type III glycogen storage disease with cirrhosis
Explanation: ***Type IV glycogen storage disease with amylopectin-like glycogen***
- The presence of **abnormal glycogen with long outer branches and fewer branch points** (amylopectin-like molecules) is the pathognomonic finding of **amylo-1,4→1,6-transglucosidase (branching enzyme)** deficiency.
- This condition characteristically leads to **progressive liver cirrhosis**, portal hypertension, and liver failure in early childhood, often without the severe hypoglycemia seen in other types.
*Type I glycogen storage disease with hepatic fibrosis*
- Caused by a deficiency in **glucose-6-phosphatase**, leading to severe **fasting hypoglycemia**, lactic acidosis, and hyperuricemia.
- The liver stores **structurally normal glycogen**, and patients typically have "doll-like" facies and renomegaly, which are not described here.
*Type III glycogen storage disease with cirrhosis*
- Caused by a deficiency in the **debranching enzyme**, leading to the accumulation of **limit dextrin** which features short, stunted outer branches rather than long ones.
- Unlike Type IV, it typically presents with **significant ketotic hypoglycemia** and the liver disease is generally less progressive and does not usually lead to early cirrhosis.
*Type VI glycogen storage disease with hepatomegaly*
- Results from a deficiency in **liver phosphorylase**, leading to hepatomegaly and mild growth retardation during childhood.
- This type presents with a **benign clinical course** and mild hypoglycemia, rather than the progressive cirrhosis and portal hypertension seen in this case.
*Type IX glycogen storage disease with liver dysfunction*
- Caused by a deficiency in **phosphorylase kinase**, which is required to activate liver phosphorylase.
- Symptoms are usually mild, including **hepatomegaly and growth delay**, and typically resolve or improve during puberty without progressing to liver failure.
Question 7: A 3-year-old boy presents with hepatomegaly, growth retardation, and fasting hypoglycemia that is less severe than his sibling who has Von Gierke disease. Laboratory studies show elevated transaminases and mild hyperlipidemia, but normal lactate and uric acid levels. Unlike his sibling, he responds to glucagon administration with increased blood glucose when fed, but not after prolonged fasting. Apply these findings to identify the enzymatic defect.
A. Debranching enzyme deficiency (Correct Answer)
B. Branching enzyme deficiency
C. Glucose-6-phosphatase deficiency
D. Liver phosphorylase deficiency
E. Phosphorylase kinase deficiency
Explanation: ***Debranching enzyme deficiency***
- **Cori disease (Type III GSD)** presents with hepatomegaly and hypoglycemia that is generally **milder than Von Gierke disease**, with a characteristic absence of **hyperuricemia and lactic acidosis** [1], [3].
- In Cori disease, the administration of **glucagon** in a fed state increases blood glucose because **phosphorylase** can still release glucose-1-phosphate from outer chains, but it fails during fasting as the **debranching enzyme** is required to bypass alpha-1,6 branch points [3].
*Glucose-6-phosphatase deficiency*
- Deficiency causes **Von Gierke disease (Type I GSD)**, which presents with severe fasting hypoglycemia, **lactic acidosis**, and **hyperuricemia**, none of which are seen in this patient [1], [3].
- Patients with this defect do not respond to **glucagon** at all because the final step of **gluconeogenesis** and **glycogenolysis** is blocked.
*Branching enzyme deficiency*
- Deficiency causes **Andersen disease (Type IV GSD)**, which typically presents with **cirrhosis**, progressive liver failure, and **hypotonia** early in life.
- It is characterized by the accumulation of **long-chain polysaccharides** (amylopectin-like) and does not typically present with significant hypoglycemia.
*Liver phosphorylase deficiency*
- Deficiency causes **Hers disease (Type VI GSD)**, which is a mild condition often presenting only with **hepatomegaly** and growth retardation.
- While it causes mild hypoglycemia, it would not show the specific differential response to **glucagon** seen in this case where it works specifically in the fed state.
*Phosphorylase kinase deficiency*
- Deficiency causes **Type IX GSD**, which is clinically similar to **Hers disease** and often presents with hepatomegaly and growth delay in early childhood [2].
- It typically follows an **X-linked** inheritance pattern and lacks the specific diagnostic nuance of the fed-state glucagon response associated with debranching enzyme defects [2].
Question 8: A 4-year-old girl presents with progressive muscle weakness, hypotonia, and cardiomegaly. Chest X-ray shows massive cardiomegaly with pulmonary congestion. Echocardiography reveals severe left ventricular hypertrophy with outflow obstruction. Electromyography shows myopathic changes, and serum creatine kinase is markedly elevated. Muscle biopsy shows vacuoles filled with glycogen and increased lysosomal acid phosphatase activity. Apply this information to determine the deficient enzyme.
A. Muscle phosphorylase
B. Debranching enzyme
C. Lysosomal alpha-1,4-glucosidase (acid maltase) (Correct Answer)
D. Glucose-6-phosphatase
E. Phosphofructokinase
Explanation: ***Lysosomal alpha-1,4-glucosidase (acid maltase)***
- Deficiency of this enzyme causes **Pompe disease (GSD type II)**, which is unique among glycogen storage diseases for its massive **cardiomegaly** and **infantile hypotonia**.
- Histology showing **glycogen-filled vacuoles** with increased **acid phosphatase activity** confirms lysosomal storage, distinguishing it from cytoplasmic glycogen defects.
*Muscle phosphorylase*
- Deficiency leads to **McArdle disease (GSD type V)**, which typically presents in teenagers or adults with **exercise-induced cramps** and **myoglobinuria**.
- It does not involve the heart or cause the severe **infantile-onset hypotonia** and cardiomegaly described in the clinical scenario.
*Debranching enzyme*
- Deficiency results in **Cori disease (GSD type III)**, presenting with **hepatomegaly**, stunted growth, and mild muscle weakness.
- While it can involve the heart in some cases, it lacks the signature **lysosomal vacuolation** and the early, massive **hypertrophic cardiomyopathy** of Pompe disease.
*Glucose-6-phosphatase*
- Deficiency causes **Von Gierke disease (GSD type I)**, characterized by severe **fasting hypoglycemia**, **hepatomegaly**, and **lactic acidosis**.
- This enzyme is absent in muscle tissue, so deficiency does not result in **myopathy** or **cardiomegaly**.
*Phosphofructokinase*
- Deficiency causes **Tarui disease (GSD type VII)**, which presents similarly to McArdle disease with **exercise intolerance** and hemolysis.
- It is not associated with **infantile cardiomegaly** or the specific **lysosomal pathology** found in the biopsy results.
Question 9: A 20-year-old man presents with muscle cramps and myoglobinuria after moderate exercise. He reports that he gets a 'second wind' if he rests briefly during exercise and then continues. Ischemic forearm exercise test shows no rise in venous lactate. Muscle biopsy reveals periodic acid-Schiff (PAS)-positive material. Apply these findings to identify the underlying enzyme deficiency.
A. Phosphofructokinase deficiency
B. Debranching enzyme deficiency
C. Muscle phosphorylase deficiency (Correct Answer)
D. Branching enzyme deficiency
E. Phosphorylase kinase deficiency
Explanation: ***Muscle phosphorylase deficiency***
- The presence of the **'second wind' phenomenon** and **exercise-induced myoglobinuria** are hallmark features of **McArdle disease** (GSD Type V), caused by a deficiency in myophosphorylase.
- An **ischemic forearm test** showing **no rise in venous lactate** combined with **PAS-positive** glycogen accumulation in muscle confirms the inability to break down glycogen into glucose-1-phosphate.
*Phosphofructokinase deficiency*
- Also known as **Tarui disease** (GSD Type VII), it presents similarly to McArdle disease but often includes **hemolytic anemia** due to enzyme deficiency in RBCs.
- Patients with this deficiency do not typically exhibit the **'second wind' phenomenon** and may experience a 'vicious circle' where glucose intake worsens symptoms.
*Debranching enzyme deficiency*
- Deficiency in **alpha-1,6-glucosidase** leads to **Cori disease** (GSD Type III), which primarily presents with **hepatomegaly** and **fasting hypoglycemia**.
- While it can involve skeletal muscle, the **ischemic forearm test** would usually show a **blunted but present lactate rise**, unlike the flat response seen here.
*Branching enzyme deficiency*
- Known as **Andersen disease** (GSD Type IV), it typically presents in infancy with **liver cirrhosis**, **failure to thrive**, and **hypotonia**.
- It results in the accumulation of **abnormal glycogen (amylopectin-like)** with fewer branch points, which is fatal at a young age and does not match the exercise-induced presentation.
*Phosphorylase kinase deficiency*
- This is most commonly an **X-linked** condition (GSD Type IX) that primarily affects the **liver**, leading to hepatomegaly and growth retardation.
- While muscle isoforms exist, the clinical picture is usually milder than McArdle disease and does not characteristically feature the **'second wind' phenomenon** or severe myoglobinuria.
Question 10: A 6-month-old infant presents with severe hypoglycemia, hepatomegaly, and lactic acidosis. Physical examination reveals a doll-like face with fat cheeks and protuberant abdomen. Laboratory studies show elevated serum lactate, uric acid, and triglycerides. A liver biopsy demonstrates excessive glycogen accumulation. Glucagon administration fails to increase blood glucose levels. Apply your knowledge to determine the most likely enzymatic defect.
A. Branching enzyme deficiency
B. Muscle phosphorylase deficiency
C. Lysosomal alpha-glucosidase deficiency
D. Glucose-6-phosphatase deficiency (Correct Answer)
E. Debranching enzyme deficiency
Explanation: ***Glucose-6-phosphatase deficiency***
- This infant presents with **Von Gierke disease (GSD Type I)**, characterized by an inability to convert **glucose-6-phosphate to glucose**, impairing both **glycogenolysis** and **gluconeogenesis**.
- Clinical hallmarks include **severe fasting hypoglycemia**, **hepatomegaly**, **lactic acidosis** (due to decreased lactate clearance), and **hyperuricemia**, with a characteristic **doll-like face** due to fat deposition.
*Muscle phosphorylase deficiency*
- Also known as **McArdle disease (GSD Type V)**, this condition affects skeletal muscle and presents with **exercise intolerance**, muscle cramps, and **myoglobinuria**.
- It does not cause **hypoglycemia** or **hepatomegaly**, as the liver enzyme remains functional.
*Debranching enzyme deficiency*
- Deficiency results in **Cori disease (GSD Type III)**, which presents with hepatomegaly and milder hypoglycemia than Type I.
- Key differentiators are the presence of **normal blood lactate levels** and the fact that **gluconeogenesis** is preserved.
*Lysosomal alpha-glucosidase deficiency*
- Known as **Pompe disease (GSD Type II)**, this is a lysosomal storage disorder that primarily causes massive **cardiomegaly** and systemic **hypotonia**.
- It typically does not present with **hypoglycemia** because cytoplasmic glycogenolysis is still functional.
*Branching enzyme deficiency*
- Known as **Andersen disease (GSD Type IV)**, it typically presents in early infancy with progressive **liver cirrhosis** and failure.
- This condition is characterized by the accumulation of **abnormal glycogen (amylopectin-like)** rather than excessive normal glycogen, and it lacks the severe lactic acidosis seen here.
