Genetic Disorders and Biochemical Pathology — MCQs

Genetic Disorders and Biochemical Pathology Indian Medical PG Practice Questions and MCQs

Question 51: What is the most common glycogen storage disorder to present in adolescents?

A. McArdle disease (Correct Answer)
B. Cori disease
C. Andersen disease
D. Von Gierke disease

Explanation: **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).

Question 52: What is true about Crigler-Najjar type II syndrome?

A. Diglucuronide deficiency (Correct Answer)
B. Recessive trait
C. Kernicterus is seen
D. Phenobarbitone is not useful

Explanation: **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.

Question 53: Hyperuricemia in von Gierke disease is due to the deficiency of which enzyme?

A. Hypoxanthine-guanine phosphoribosyl transferase
B. Glucose-6-phosphatase (Correct Answer)
C. Xanthine oxidase
D. Adenosine deaminase

Explanation: ### Explanation **Correct Option: B. Glucose-6-phosphatase** **Mechanism of Hyperuricemia in von Gierke Disease (GSD Type I):** The deficiency of **Glucose-6-phosphatase** prevents the conversion of Glucose-6-Phosphate (G6P) into free glucose. This leads to an intracellular accumulation of G6P, which is then diverted into alternative metabolic pathways: 1. **Increased Pentose Phosphate Pathway (PPP):** Excess G6P enters the PPP, leading to increased production of **Ribose-5-phosphate**. This provides the substrate for **PRPP (Phosphoribosyl pyrophosphate)** synthesis, which accelerates *de novo* purine synthesis. The subsequent breakdown of these excess purines results in hyperuricemia. 2. **Lactic Acid Competition:** Impaired gluconeogenesis leads to chronic lactic acidosis. Lactic acid competes with uric acid for the same secretory transport mechanism in the renal tubules, decreasing uric acid excretion and further elevating serum levels. --- ### Analysis of Incorrect Options: * **A. HGPRT:** Deficiency causes **Lesch-Nyhan Syndrome**. While it causes hyperuricemia due to failure of the purine salvage pathway, it is not the enzyme deficient in von Gierke disease. * **C. Xanthine oxidase:** This enzyme converts hypoxanthine/xanthine to uric acid. Its deficiency (Xanthinuria) or inhibition (by Allopurinol) actually **lowers** uric acid levels. * **D. Adenosine deaminase:** Deficiency causes **Severe Combined Immunodeficiency (SCID)** due to the accumulation of dATP, which is toxic to lymphocytes. --- ### NEET-PG High-Yield Pearls: * **Clinical Tetrad of von Gierke:** Hypoglycemia (fasting), Hyperlactatemia, Hyperuricemia, and Hyperlipidemia (doll-like facies and hepatomegaly). * **Hyperuricemia Management:** Often presents as "Gouty arthritis" in older children; managed with Allopurinol. * **Biochemical Hallmark:** Unlike other GSDs, von Gierke presents with significant **lactic acidosis** because G6P cannot leave the liver and enters glycolysis instead.

Question 54: Which of the following is not included under Garrod's tetrad?

A. Albinism
B. Pentosuria
C. Phenylketonuria (Correct Answer)
D. Cystinuria

Explanation: **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.

Question 55: 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?

A. Glucose 6-phosphatase
B. Muscle phosphorylase
C. Liver phosphorylase
D. Acid maltase (Correct Answer)

Explanation: ### 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.

Question 56: Nitisinone is an example of which of the following therapeutic approaches?

A. Enzyme activation therapy
B. Enzyme replacement therapy
C. Substrate addition therapy
D. Substrate reduction therapy (Correct Answer)

Explanation: **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.

Question 57: The level of alpha fetoprotein is raised in all of the following conditions except?

A. Cirrhosis of the liver
B. Hepatocellular carcinoma
C. Yolk sac tumor
D. Dysgerminoma (Correct Answer)

Explanation: **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.

Question 58: What is true about Gilbert syndrome?

A. Conjugated hyperbilirubinemia
B. Starvation improves the condition
C. Phenobarbitone improves the condition (Correct Answer)
D. Life-threatening

Explanation: **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.

Question 59: All of the following are X-linked except?

A. G-6 PD deficiency
B. Hemophilia - A
C. Von Willebrandt's disease (Correct Answer)
D. Fragile X syndrome

Explanation: **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.

Question 60: Which X-linked recessive disease in males presents with a clotting defect?

A. Hemophilia A (Correct Answer)
B. Von Willebrand disease
C. Idiopathic Thrombocytopenic Purpura (ITP)
D. None of the above

Explanation: ### 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.

Practice by Chapter

Single Gene Disorders

Practice Questions

Biochemical Diagnosis of Genetic Disorders

Practice Questions

Inborn Errors of Metabolism

Practice Questions

Lysosomal Storage Diseases

Practice Questions

Glycogen Storage Diseases

Practice Questions

Disorders of Lipoprotein Metabolism

Practice Questions

Disorders of Purine and Pyrimidine Metabolism

Practice Questions

Hemoglobinopathies

Practice Questions

Porphyrias

Practice Questions

Biochemical Markers for Disease Diagnosis

Practice Questions

Newborn Screening for Genetic Disorders

Practice Questions

Enzyme Replacement Therapy

Practice Questions

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