Genetic Disorders and Biochemical Pathology Practice Questions

Q: What is the most common glycogen storage disorder to present in adolescents?

McArdle disease. **Explanation:** The correct answer is **McArdle disease (GSD Type V)**. The primary reason it presents in adolescence or early adulthood is that it affects **muscle glycogen phosphorylase (myophosphorylase)**, an enzyme required only during high-intensity anaerobic exercise. Unlike hepatic glycogen storage diseases that manifest in infancy with systemic metabolic crises (hypoglycemia), McArdle disease is a purely myogenic disorder. Symptoms typically emerge when the patient begins engaging in more strenuous physical activity during teenage years. **Why the other options are incorrect:** * **Von Gierke disease (Type I):** This is the most common GSD overall but presents in **infancy** (3–6 months) with severe fasting hypoglycemia, hepatomegaly, and doll-like facies. * **Cori disease (Type III):** Presents in **early childhood** with hepatomegaly and growth retardation. While it involves muscle weakness, the metabolic symptoms appear much earlier than adolescence. * **Andersen disease (Type IV):** A very severe form presenting in **infancy** with failure to thrive and progressive liver cirrhosis, often leading to death by age 2. **NEET-PG High-Yield Pearls:** * **Clinical Triad:** Exercise intolerance, muscle cramps, and **myoglobinuria** (burgundy-colored urine) after strenuous exercise. * **Second-Wind Phenomenon:** A pathognomonic feature where symptoms improve after a short period of exercise due to the mobilization of fatty acids and increased blood flow. * **Biochemical Marker:** Failure of blood lactate to rise during an ischemic exercise test (flat lactate curve). * **Key Enzyme:** Myophosphorylase (Muscle glycogen phosphorylase).

Q: What is true about Crigler-Najjar type II syndrome?

Diglucuronide deficiency. **Explanation:** Crigler-Najjar Syndrome (CNS) is caused by a deficiency of the enzyme **UDP-glucuronosyltransferase (UGT1A1)**, which is responsible for conjugating bilirubin with glucuronic acid in the liver. **1. Why Option A is Correct:** In Crigler-Najjar Type II (Arias Syndrome), there is a partial deficiency of UGT1A1 (enzyme activity is usually <10% of normal). Because the enzyme is present but severely limited, it can only perform the first step of conjugation effectively. This results in the formation of **bilirubin monoglucuronide**, but there is a significant **deficiency of bilirubin diglucuronide** in the bile. **2. Why the other options are incorrect:** * **Option B (Recessive trait):** Unlike Type I, which is strictly autosomal recessive, Type II is typically inherited as an **autosomal dominant** trait with variable penetrance (though some cases show recessive patterns). * **Option C (Kernicterus):** Kernicterus is a hallmark of **Type I** (complete enzyme absence). In Type II, serum bilirubin levels are lower (usually 6–20 mg/dL), and **kernicterus is rare**, typically occurring only during periods of severe stress or illness. * **Option D (Phenobarbitone):** Phenobarbitone is a potent inducer of the UGT1A1 enzyme. It is **highly effective** in Type II, significantly reducing serum bilirubin levels. It has no effect in Type I because the enzyme is completely absent. **High-Yield Clinical Pearls for NEET-PG:** * **Type I vs. Type II:** The most definitive clinical test to differentiate the two is the **Phenobarbitone response test** (Positive in Type II; Negative in Type I). * **Bile Color:** Bile is colorless in Type I (no conjugated bilirubin) but pigmented in Type II. * **Gilbert Syndrome:** The mildest form of UGT1A1 deficiency (enzyme activity ~30% of normal), often asymptomatic until triggered by fasting or stress.

Q: Which of the following is not included under Garrod's tetrad?

Phenylketonuria. **Explanation** Sir Archibald Garrod, known as the "Father of Inborn Errors of Metabolism," first proposed the concept that certain diseases result from a genetic deficiency of a specific enzyme. In 1902, he described a group of four conditions collectively known as **Garrod’s Tetrad**. **Why Phenylketonuria (PKU) is the correct answer:** Phenylketonuria is **not** part of the original tetrad. Although PKU is a classic inborn error of metabolism (deficiency of Phenylalanine Hydroxylase), it was discovered much later, in 1934, by Ivar Asbjørn Følling. **Analysis of Garrod’s Tetrad (Incorrect Options):** The four conditions originally identified by Garrod are: 1. **Alkaptonuria (Option A):** The first one described; characterized by a deficiency of Homogentisate Oxidase leading to black urine. 2. **Albinism (Option A):** Specifically Oculocutaneous Albinism, involving a defect in Tyrosinase. 3. **Pentosuria (Option B):** A benign condition involving the excretion of L-xylulose due to a deficiency of L-xylulose reductase. 4. **Cystinuria (Option D):** A transport defect of COAL (Cystine, Ornithine, Arginine, Lysine) in the renal tubules, leading to hexagonal cystine stones. **High-Yield Clinical Pearls for NEET-PG:** * **Alkaptonuria:** Look for "Ochronosis" (pigmentation of connective tissue) and arthritis in clinical vignettes. * **Pentosuria:** Often confused with Diabetes Mellitus because it gives a positive Benedict’s test (reducing sugar), but it shows a negative Glucose Oxidase test. It is common in Ashkenazi Jews. * **Cystinuria:** Diagnosed via the **Cyanide-Nitroprusside test**. * **Garrod’s Hypothesis:** He coined the term "Inborn Errors of Metabolism" and was the first to suggest the "one gene, one enzyme" relationship.

Q: An infant presents to OPD with signs of heart failure. On examination, there is hypotonia, hepatomegaly, and cardiomegaly. ECG shows tall QRS complex and a short PR interval. Which enzyme is deficient in this condition?

Acid maltase. ### Explanation The clinical presentation of **cardiomegaly, hepatomegaly, and hypotonia** in an infant, combined with pathognomonic ECG findings (**short PR interval and tall QRS complexes**), is a classic description of **Pompe Disease (Glycogen Storage Disease Type II)**. **1. Why Acid Maltase is Correct:** Pompe disease is caused by a deficiency of **Lysosomal $\alpha$-1,4-glucosidase (Acid Maltase)**. Unlike other glycogen storage diseases (GSDs), this is a **lysosomal storage disorder**. Acid maltase is responsible for breaking down glycogen within lysosomes. Its deficiency leads to massive accumulation of glycogen in the heart, skeletal muscle, and liver. The cardiac involvement is the hallmark, leading to hypertrophic cardiomyopathy and early heart failure. **2. Why Other Options are Incorrect:** * **Glucose 6-phosphatase (Option A):** Deficient in **Von Gierke Disease (GSD I)**. While it presents with hepatomegaly and hypoglycemia, it **does not** involve the heart or cause skeletal muscle hypotonia. * **Muscle phosphorylase (Option B):** Deficient in **McArdle Disease (GSD V)**. This presents in adolescence/adulthood with exercise intolerance and cramps; it does not cause infantile heart failure. * **Liver phosphorylase (Option C):** Deficient in **Hers Disease (GSD VI)**. This is a milder condition presenting with hepatomegaly and growth retardation, without cardiac involvement. **3. High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** "Pompe trashes the **Pump** (Heart), **Liver**, and **Muscle**." * **ECG Hallmark:** Short PR interval due to accelerated conduction through glycogen-laden myocardium and massive QRS voltage (biventricular hypertrophy). * **Histology:** PAS-positive material in lysosomes. * **Treatment:** Enzyme Replacement Therapy (ERT) with Alglucosidase alfa.

Q: Nitisinone is an example of which of the following therapeutic approaches?

Substrate reduction therapy. **Explanation:** **Nitisinone** (NTBC) is the primary treatment for **Hereditary Tyrosinemia Type I (HT-1)**. HT-1 is caused by a deficiency of the enzyme *Fumarylacetoacetate Hydrolase (FAH)*, the final step in tyrosine catabolism. This deficiency leads to the accumulation of toxic metabolites like **succinylacetone**, which causes severe liver failure, renal tubular dysfunction, and hepatocellular carcinoma. **1. Why Substrate Reduction Therapy (SRT) is correct:** Nitisinone acts as a potent inhibitor of **4-hydroxyphenylpyruvate dioxygenase (4-HPPD)**, an enzyme upstream in the tyrosine degradation pathway. By blocking this enzyme, Nitisinone prevents the formation of maleylacetoacetate and fumarylacetoacetate, thereby reducing the production of the toxic substrate succinylacetone. This "metabolic bypass" shifts the pathology from a lethal condition to a manageable one (Tyrosinemia Type III-like state). **2. Why other options are incorrect:** * **Enzyme Activation:** This involves using small molecules to increase the residual activity of a mutant enzyme (e.g., Vitamin B6 for Homocystinuria). Nitisinone *inhibits* rather than activates. * **Enzyme Replacement Therapy (ERT):** This involves administering the functional recombinant enzyme intravenously (e.g., Alglucerase for Gaucher disease). * **Substrate Addition:** This involves providing the end-product that the body cannot produce (e.g., Citrulline in certain Urea Cycle Disorders). **High-Yield Clinical Pearls for NEET-PG:** * **Biochemical Marker:** Succinylacetone in urine is pathognomonic for Tyrosinemia Type I. * **Dietary Management:** Patients on Nitisinone must follow a **low-tyrosine and low-phenylalanine diet** to prevent corneal crystals and neurological issues caused by secondary hypertyrosinemia. * **Other SRT Examples:** **Miglustat** (used in Gaucher disease) inhibits glucosylceramide synthase, another classic example of SRT.

Q: The level of alpha fetoprotein is raised in all of the following conditions except?

Dysgerminoma. **Explanation:** Alpha-fetoprotein (AFP) is a glycoprotein synthesized primarily by the fetal liver and yolk sac. In adults, elevated levels serve as a crucial tumor marker or an indicator of specific regenerative/inflammatory liver processes. **Why Dysgerminoma is the correct answer:** Dysgerminoma is the female counterpart of the male seminoma. It is a germ cell tumor characterized by the proliferation of primitive germ cells that have not differentiated into yolk sac or embryonal structures. Consequently, it **does not produce AFP**. The characteristic markers for Dysgerminoma are **LDH** (Lactate Dehydrogenase) and occasionally **hCG** (if syncytiotrophoblastic giant cells are present). **Analysis of Incorrect Options:** * **Cirrhosis of the liver:** AFP can be mildly to moderately elevated in non-malignant chronic liver diseases like cirrhosis and hepatitis due to active hepatocyte regeneration. * **Hepatocellular carcinoma (HCC):** AFP is the classic tumor marker for HCC. Levels >400–500 ng/mL in a high-risk patient are highly suggestive of this malignancy. * **Yolk sac tumor (Endodermal Sinus Tumor):** This tumor histologically mimics the fetal yolk sac; therefore, it produces **extremely high levels of AFP**. It is the most common testicular tumor in children under 3 years. **NEET-PG High-Yield Pearls:** 1. **AFP in Pregnancy:** Raised in Neural Tube Defects (NTDs) and abdominal wall defects (omphalocele/gastroschisis); **Decreased** in Down Syndrome (Trisomy 21). 2. **Schiller-Duval Bodies:** Pathognomonic histological finding in Yolk Sac Tumors. 3. **Mixed Germ Cell Tumors:** If a patient has a "Dysgerminoma" but the AFP is elevated, it indicates a mixed component (likely Yolk Sac). 4. **Rule of Thumb:** AFP = Liver (HCC/Cirrhosis) or Yolk Sac derivatives.

Q: What is true about Gilbert syndrome?

Phenobarbitone improves the condition. **Explanation:** **Gilbert Syndrome** is a common, benign autosomal recessive condition characterized by reduced activity of the enzyme **UDP-glucuronosyltransferase (UGT1A1)**, typically to about 30% of normal levels. 1. **Why Option C is Correct:** **Phenobarbitone** is a potent inducer of microsomal enzymes, including UGT1A1. By inducing the expression of the remaining functional enzyme, it enhances the conjugation of bilirubin, thereby lowering serum bilirubin levels and improving the clinical jaundice. 2. **Why Other Options are Incorrect:** * **Option A:** It is characterized by **Unconjugated hyperbilirubinemia**. Since the defect lies in the conjugation process in the liver, unconjugated bilirubin accumulates. * **Option B:** **Starvation (fasting)**, dehydration, stress, and illness actually **exacerbate** the condition. Fasting reduces the availability of UDP-glucuronic acid and decreases hepatic uptake, leading to a transient rise in bilirubin. * **Option D:** It is a **benign, asymptomatic** condition. It does not lead to liver damage or kernicterus and has a normal life expectancy. **High-Yield Clinical Pearls for NEET-PG:** * **Genetic Defect:** Most commonly due to a TATAA box mutation in the *UGT1A1* gene. * **Trigger Factors:** Jaundice typically appears during periods of stress, fasting, or strenuous exercise. * **Diagnosis:** Suspected when there is isolated unconjugated hyperbilirubinemia (<3 mg/dL) with normal liver enzymes (ALT/AST) and no evidence of hemolysis. * **Comparison:** Unlike **Crigler-Najjar Syndrome Type I** (zero enzyme activity) and **Type II** (Arias Syndrome, <10% activity), Gilbert syndrome is the mildest form of hereditary glucuronidation defects.

Q: All of the following are X-linked except?

Von Willebrandt's disease. **Explanation:** The correct answer is **Von Willebrand Disease (vWD)** because it is primarily an **autosomal dominant** disorder (Type 1 and 2) or, less commonly, autosomal recessive (Type 3). It is caused by mutations in the *VWF* gene located on **chromosome 12**, not the X chromosome. **Analysis of Options:** * **A. G-6PD Deficiency:** This is a classic **X-linked recessive** enzymopathy. It leads to neonatal jaundice or acute hemolytic anemia triggered by oxidative stress (e.g., fava beans, primaquine). * **B. Hemophilia A:** This is an **X-linked recessive** bleeding disorder caused by a deficiency of Factor VIII. It predominantly affects males, while females are typically asymptomatic carriers. * **D. Fragile X Syndrome:** This is an **X-linked dominant** condition (with variable expressivity) caused by CGG trinucleotide repeat expansion in the *FMR1* gene. It is the most common cause of inherited intellectual disability. **High-Yield Clinical Pearls for NEET-PG:** * **vWD vs. Hemophilia:** vWD is the **most common inherited bleeding disorder** overall. Unlike Hemophilia (which affects deep tissues/joints), vWD typically presents with mucosal bleeding (epistaxis, menorrhagia) and a prolonged bleeding time. * **Mnemonic for X-linked Recessive Disorders:** "**G**o **H**ome **F**or **D**inner **M**y **L**ittle **B**oy" (**G**6PD, **H**emophilia A/B, **F**abry’s, **D**uchenne Muscular Dystrophy, **M**enke’s, **L**esch-Nyhan, **B**ruton’s Agammaglobulinemia). * **Exception:** While most Hemophilias are X-linked, **Hemophilia C** (Factor XI deficiency) is autosomal recessive.

Q: Which X-linked recessive disease in males presents with a clotting defect?

Hemophilia A. ### Explanation **1. Why Hemophilia A is the Correct Answer:** Hemophilia A is a classic **X-linked recessive** bleeding disorder caused by a deficiency or dysfunction of **Clotting Factor VIII**. Because it is X-linked, it primarily affects males, while females are typically asymptomatic carriers. Factor VIII is a crucial cofactor in the intrinsic pathway of the coagulation cascade; its deficiency leads to a failure in fibrin clot formation, presenting clinically as deep tissue bleeds, hemarthrosis (bleeding into joints), and prolonged bleeding after trauma or surgery. **2. Why the Other Options are Incorrect:** * **B. Von Willebrand Disease (vWD):** This is the most common inherited bleeding disorder, but it follows an **Autosomal Dominant** inheritance pattern (most types). It involves a deficiency or defect in von Willebrand Factor (vWF), affecting platelet adhesion rather than just the clotting cascade. * **C. Idiopathic Thrombocytopenic Purpura (ITP):** This is an **acquired autoimmune** condition where antibodies are directed against platelets, leading to their destruction. It is not a genetic clotting factor defect. * **D. None of the above:** Incorrect, as Hemophilia A fits all criteria mentioned in the stem. **3. High-Yield Clinical Pearls for NEET-PG:** * **Hemophilia B (Christmas Disease):** Also X-linked recessive, but caused by **Factor IX** deficiency. * **Lab Findings:** In Hemophilia, the **aPTT is prolonged**, while PT, Bleeding Time (BT), and Platelet count remain **normal**. * **Mixing Study:** A hallmark of Hemophilia is that the prolonged aPTT **corrects** when the patient's plasma is mixed with normal plasma (indicating a factor deficiency rather than an inhibitor). * **vWD vs. Hemophilia:** vWD often presents with an increased Bleeding Time (BT) and skin/mucosal bleeds (epistaxis, menorrhagia), whereas Hemophilia presents with deep-seated bleeds.

Question 1

What is the most common glycogen storage disorder to present in adolescents?

Accepted Answer

McArdle disease

Answer

Cori disease

Answer

Andersen disease

Answer

Von Gierke disease

Question 2

What is true about Crigler-Najjar type II syndrome?

Accepted Answer

Diglucuronide deficiency

Answer

Recessive trait

Answer

Kernicterus is seen

Answer

Phenobarbitone is not useful

Question 3

Hyperuricemia in von Gierke disease is due to the deficiency of which enzyme?

Accepted Answer

Glucose-6-phosphatase

Answer

Hypoxanthine-guanine phosphoribosyl transferase

Answer

Xanthine oxidase

Answer

Adenosine deaminase

Question 4

Which of the following is not included under Garrod's tetrad?

Accepted Answer

Phenylketonuria

Answer

Albinism

Answer

Pentosuria

Answer

Cystinuria

Question 5

An infant presents to OPD with signs of heart failure. On examination, there is hypotonia, hepatomegaly, and cardiomegaly. ECG shows tall QRS complex and a short PR interval. Which enzyme is deficient in this condition?

Accepted Answer

Acid maltase

Answer

Glucose 6-phosphatase

Answer

Muscle phosphorylase

Answer

Liver phosphorylase

Question 6

Nitisinone is an example of which of the following therapeutic approaches?

Accepted Answer

Substrate reduction therapy

Answer

Enzyme activation therapy

Answer

Enzyme replacement therapy

Answer

Substrate addition therapy

Question 7

The level of alpha fetoprotein is raised in all of the following conditions except?

Accepted Answer

Dysgerminoma

Answer

Cirrhosis of the liver

Answer

Hepatocellular carcinoma

Answer

Yolk sac tumor

Question 8

What is true about Gilbert syndrome?

Accepted Answer

Phenobarbitone improves the condition

Answer

Conjugated hyperbilirubinemia

Answer

Starvation improves the condition

Answer

Life-threatening

Question 9

All of the following are X-linked except?

Accepted Answer

Von Willebrandt's disease

Answer

G-6 PD deficiency

Answer

Hemophilia - A

Answer

Fragile X syndrome

Question 10

Which X-linked recessive disease in males presents with a clotting defect?

Accepted Answer

Hemophilia A

Answer

Von Willebrand disease

Answer

Idiopathic Thrombocytopenic Purpura (ITP)

Answer

None of the above

Genetic Disorders and Biochemical Pathology — MCQs

Genetic Disorders and Biochemical Pathology — MCQs

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