Genetic Disorders and Biochemical Pathology — MCQs

A 29-year-old man has had angina for the past year with a family history of cardiovascular disease. His blood pressure is 120/80 mm Hg, total serum cholesterol is 185 mg/dL, and glucose is 85 mg/dL. A mutation involving a gene encoding for which of the following is most likely present in this man's family?

Q309Medium

An albino girl is married to a normal boy. What are the chances of their having an affected child and what are the chances of their children being carriers?

Q310Medium

The hallmarks of type Ia glycogen storage disease are all of the following except?

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

Question 301: Accumulation of sphingomyelin in the spleen and liver is found in which condition?

A. Gaucher's disease
B. Obstructive jaundice
C. Von Gierke's disease
D. Niemann-Pick disease (Correct Answer)

Explanation: ### Explanation **Niemann-Pick Disease (Correct Answer):** Niemann-Pick disease (specifically Types A and B) is a lysosomal storage disorder caused by a deficiency of the enzyme **Acid Sphingomyelinase**. This deficiency leads to the progressive accumulation of **sphingomyelin** (a major component of cell membranes and myelin sheaths) within the lysosomes of macrophages. These lipid-laden macrophages, known as **"Foam cells,"** infiltrate the reticuloendothelial system, resulting in massive **hepatosplenomegaly**, lymphadenopathy, and often a "cherry-red spot" on the macula. **Why the other options are incorrect:** * **Gaucher’s Disease:** This is the most common lysosomal storage disorder, caused by a deficiency of **$\beta$-Glucocerebrosidase**. It leads to the accumulation of **glucocerebroside**. While it also presents with hepatosplenomegaly, the characteristic histological finding is the "Gaucher cell" (wrinkled tissue paper appearance), not sphingomyelin accumulation. * **Obstructive Jaundice:** This is a clinical condition resulting from the blockage of bile flow. While it can cause hepatomegaly and elevated cholesterol/lipids in the blood, it does not involve the genetic deficiency of lysosomal enzymes or the specific accumulation of sphingomyelin. * **Von Gierke’s Disease (GSD Type I):** This is a glycogen storage disease caused by a deficiency of **Glucose-6-Phosphatase**. It leads to the accumulation of **glycogen** (not lipids) in the liver and kidneys, presenting with severe hypoglycemia and a "doll-like" face. **High-Yield NEET-PG Pearls:** * **Histology:** Look for **"Foam cells"** (lipid-laden macrophages with a vacuolated appearance) in Niemann-Pick. * **Genetics:** Autosomal Recessive inheritance. * **Type A vs. B:** Type A is the infantile, neuropathic form (early death); Type B is the non-neuropathic form (survival into adulthood). * **Mnemonic:** "No-man picks (Niemann-Pick) his nose with a **foamy** finger" (Foam cells).

Question 302: Enzyme imiglucerase (Cerezyme) is used in the treatment of which condition?

A. Gaucher's disease (Correct Answer)
B. Galactosemia
C. Niemann Pick disease
D. Trans-maxillary approach

Explanation: **Explanation:** **Imiglucerase (Cerezyme)** is a recombinant form of the human lysosomal enzyme **beta-glucocerebrosidase**. It is the mainstay of **Enzyme Replacement Therapy (ERT)** for **Gaucher’s disease**, the most common lysosomal storage disorder. 1. **Why Gaucher’s Disease is Correct:** Gaucher’s disease is caused by a deficiency of the enzyme glucocerebrosidase, leading to the accumulation of glucosylceramide (glucocerebroside) in macrophages. These "Gaucher cells" infiltrate the bone marrow, liver, and spleen. Imiglucerase replaces the deficient enzyme, reducing organomegaly and improving hematological parameters. 2. **Why Other Options are Incorrect:** * **Galactosemia:** This is a metabolic disorder of carbohydrate metabolism (deficiency of GALT, GALK, or GALE). Treatment is dietary restriction of galactose/lactose, not ERT. * **Niemann-Pick Disease:** This is caused by a deficiency of **sphingomyelinase** (Types A and B). While ERT (Olipudase alfa) exists for Type B, Imiglucerase is specific only to the glucocerebrosidase deficiency in Gaucher’s. * **Trans-maxillary approach:** This is a surgical technique (often used for skull base tumors), not a biochemical condition or a target for pharmacological therapy. **High-Yield Clinical Pearls for NEET-PG:** * **Gaucher Cells:** Described as having a **"wrinkled paper"** or "crumpled silk" cytoplasm. * **Clinical Triad:** Hepatosplenomegaly, bone pain/crises (Erlenmeyer flask deformity of femur), and cytopenias. * **Biochemical Marker:** Elevated levels of **Chitotriosidase** are used to monitor disease activity and response to ERT. * **Alternative Treatment:** **Miglustat** or **Eliglustat** (Substrate Reduction Therapy) can be used for patients who cannot tolerate ERT.

Question 303: In all the following metabolic disorders, clinical features are seen due to accumulation of toxic compounds EXCEPT?

A. Phenylketonuria
B. Maple syrup urine disease
C. Urea cycle defects
D. Mitochondrial disorders (Correct Answer)

Explanation: ### Explanation The fundamental distinction in metabolic pathology lies between **"Intoxication-type"** disorders and **"Energy-deficiency"** disorders. **1. Why Mitochondrial Disorders is the Correct Answer:** Mitochondrial disorders (e.g., MELAS, Leigh syndrome) primarily result from a **failure in energy production (ATP)** rather than the accumulation of a specific upstream toxic metabolite. The clinical features—such as encephalopathy, myopathy, and lactic acidosis—arise because tissues with high metabolic demands (brain, muscle, heart) cannot meet their energy requirements due to defects in the respiratory chain or oxidative phosphorylation. **2. Why the Other Options are Incorrect:** * **Phenylketonuria (PKU):** Caused by a deficiency of phenylalanine hydroxylase. Clinical features (intellectual disability, musty odor) are due to the **toxic accumulation of phenylalanine** and its metabolites (phenylpyruvate). * **Maple Syrup Urine Disease (MSUD):** Caused by a defect in the Branched-Chain Alpha-Keto Acid Dehydrogenase complex. Symptoms (seizures, encephalopathy) are due to the **toxic buildup of Leucine, Isoleucine, and Valine.** * **Urea Cycle Defects:** Defects in enzymes like Ornithine Transcarbamylase (OTC) lead to the **toxic accumulation of Ammonia**, which is highly neurotoxic and causes cerebral edema. ### NEET-PG High-Yield Pearls: * **Intoxication Disorders:** Characterized by a "symptom-free interval" after birth followed by acute crisis (vomiting, coma) triggered by protein intake. (Includes: Organic acidemias, Urea cycle defects, Galactosemia). * **Energy Deficiency Disorders:** Often present with hypoglycemia, hypotonia, and cardiomyopathy. (Includes: Mitochondrial disorders, Fatty acid oxidation defects, Glycogen storage diseases). * **Mnemonic:** In **M**itochondrial diseases, the **M**achinery (ATP production) is broken; in others, the **P**ipe is clogged (Toxic buildup).

Question 304: What is the most common chromosomal defect?

A. Down syndrome (Correct Answer)
B. Turner syndrome
C. Edwards syndrome
D. Patau syndrome

Explanation: **Explanation:** **Down Syndrome (Trisomy 21)** is the most common chromosomal abnormality among live-born infants. It occurs due to the presence of an extra copy of chromosome 21, most frequently resulting from **meiotic non-disjunction** (95% of cases), which is strongly associated with advanced maternal age. Its prevalence is approximately 1 in 700 to 800 live births, making it significantly more frequent than other autosomal or sex chromosome aneuploidies. **Analysis of Incorrect Options:** * **Turner Syndrome (45, XO):** This is the most common sex chromosome abnormality in females, but its incidence (1 in 2,500 live births) is much lower than Down syndrome. Notably, it is the most common cause of spontaneous abortion due to chromosomal defects. * **Edwards Syndrome (Trisomy 18):** This is the second most common autosomal trisomy. However, it is much rarer (1 in 5,000–8,000 births) and has a very high mortality rate in utero and infancy. * **Patau Syndrome (Trisomy 13):** This is the least common of the three viable autosomal trisomies (1 in 10,000–16,000 births) and presents with the most severe clinical phenotype, including midline defects like holoprosencephaly. **High-Yield Clinical Pearls for NEET-PG:** * **Most common cause:** Meiotic non-disjunction (maternal origin). * **Second most common cause:** Robertsonian translocation (usually involving chromosomes 14 and 21). * **Screening:** First-trimester screening shows **increased nuchal translucency**, **decreased PAPP-A**, and **increased β-hCG**. * **Quadruple Test (Second Trimester):** Characterized by **low AFP**, **low Estriol**, **high hCG**, and **high Inhibin A**.

Question 305: Which of the following is NOT an X-linked disease?

A. Androgen insensitivity syndrome
B. Duchenne muscular dystrophy
C. Hemophilia
D. Wilson's disease (Correct Answer)

Explanation: **Explanation:** The correct answer is **Wilson’s disease** because it follows an **Autosomal Recessive** inheritance pattern, not X-linked. **1. Why Wilson’s Disease is the correct answer:** Wilson’s disease is caused by mutations in the **ATP7B gene** located on **Chromosome 13**. This defect leads to impaired biliary copper excretion and failure to incorporate copper into ceruloplasmin, resulting in toxic copper accumulation in the liver, brain (basal ganglia), and cornea (Kayser-Fleischer rings). Since the gene is on an autosome, it affects males and females equally. **2. Why the other options are incorrect:** * **Androgen Insensitivity Syndrome (AIS):** This is an **X-linked recessive** condition caused by mutations in the androgen receptor gene on the X chromosome. Affected individuals are genotypically male (46, XY) but phenotypically female. * **Duchenne Muscular Dystrophy (DMD):** This is a classic **X-linked recessive** disorder caused by a mutation in the *Dystrophin* gene (the largest known human gene), located on the X chromosome. * **Hemophilia:** Both Hemophilia A (Factor VIII deficiency) and Hemophilia B (Factor IX deficiency) are inherited in an **X-linked recessive** manner. **High-Yield Clinical Pearls for NEET-PG:** * **Wilson vs. Menkes:** While Wilson’s is Autosomal Recessive (ATP7B), **Menkes disease** (kinky hair syndrome) is **X-linked Recessive** (ATP7A). * **Common X-linked Recessive Mnemonic:** "**G**o **C**herish **H**is **D**iamonds" (**G**6PD deficiency, **C**olor blindness, **H**emophilia/Hunter syndrome, **D**MD). * **Wilson’s Diagnosis:** Look for low serum ceruloplasmin, increased urinary copper, and the "Giant Panda" sign on brain MRI.

Question 306: A genetic disorder appears in three consecutive generations of a family without any sex predilection. Phenotypically normal family members have healthy offspring. What is the pattern of inheritance of this disorder?

A. Autosomal recessive
B. Autosomal dominant (Correct Answer)
C. Mitochondrial inheritance
D. Uniparental disomy

Explanation: ### Explanation **1. Why Autosomal Dominant is Correct:** The pedigree described follows the classic criteria for **Autosomal Dominant (AD)** inheritance: * **Vertical Transmission:** The disorder appears in every generation ("three consecutive generations") without skipping. This indicates that a single mutant allele is sufficient to cause the phenotype. * **No Sex Predilection:** The disorder affects males and females equally, pointing toward an **autosomal** rather than a sex-linked chromosome. * **Normal Parents have Normal Offspring:** In AD inheritance, phenotypically normal individuals do not carry the mutant allele (genotype *aa*) and therefore cannot pass the disease to their children. **2. Why Other Options are Incorrect:** * **Autosomal Recessive (AR):** These disorders typically show **horizontal transmission** (affecting siblings in one generation). They often skip generations, and phenotypically normal parents (carriers) can have affected offspring. * **Mitochondrial Inheritance:** This follows a **maternal inheritance** pattern. An affected mother passes the trait to *all* her children, but an affected father passes it to *none*. * **Uniparental Disomy (UPD):** This occurs when a person receives two copies of a chromosome from one parent and none from the other (e.g., Prader-Willi or Angelman syndromes). It does not follow a predictable multigenerational vertical pattern. **3. High-Yield Clinical Pearls for NEET-PG:** * **Key AD Feature:** Every affected person usually has at least one affected parent. * **Exceptions to the Rule:** Watch for terms like **"Reduced Penetrance"** (has the gene but not the disease) or **"Variable Expressivity"** (severity differs among family members), which can complicate AD pedigrees. * **Common AD Disorders:** Huntington’s disease, Marfan syndrome, Neurofibromatosis (NF1/NF2), and Familial Hypercholesterolemia. * **Mnemonic:** AD disorders often involve **structural proteins** (e.g., Collagen in Osteogenesis Imperfecta), whereas AR disorders often involve **enzyme deficiencies**.

Question 307: For which glycogen storage disease, enzyme replacement therapy is available?

A. Glycogen storage disease type I
B. Glycogen storage disease type II (Correct Answer)
C. Glycogen storage disease type III
D. Glycogen storage disease type IV

Explanation: **Explanation:** **Glycogen Storage Disease Type II (Pompe Disease)** is the correct answer because it is the only GSD that is also classified as a **lysosomal storage disorder**. It is caused by a deficiency of **Acid α-1,4-glucosidase (Acid Maltase)**, the enzyme responsible for breaking down glycogen within lysosomes. Since the pathology involves a single lysosomal enzyme deficiency, it is amenable to **Enzyme Replacement Therapy (ERT)**. The FDA-approved recombinant human enzyme **Alglucosidase alfa** (and newer generations like Avalglucosidase alfa) is used to improve cardiac and skeletal muscle function in these patients. **Analysis of Incorrect Options:** * **GSD Type I (von Gierke Disease):** Caused by Glucose-6-Phosphatase deficiency. It is a metabolic pathway defect in the cytosol/ER of hepatocytes. Management focuses on dietary prevention of hypoglycemia (e.g., uncooked cornstarch). * **GSD Type III (Cori Disease):** Caused by Debranching enzyme deficiency. Management is primarily nutritional (high protein, frequent meals). * **GSD Type IV (Andersen Disease):** Caused by Branching enzyme deficiency, leading to the accumulation of abnormal glycogen (amylopectin-like). Treatment is supportive, often requiring liver transplantation. **High-Yield Clinical Pearls for NEET-PG:** * **Pompe is Unique:** It is the only GSD with **normal blood glucose levels** (no hypoglycemia) because cytosolic glycogenolysis remains intact. * **Clinical Triad:** Cardiomegaly (massive), hypotonia ("floppy baby"), and early death from heart failure. * **Histology:** PAS-positive material in lysosomes. * **Mnemonic:** "Pompe trashes the **Pump** (heart)."

Question 308: A 29-year-old man has had angina for the past year with a family history of cardiovascular disease. His blood pressure is 120/80 mm Hg, total serum cholesterol is 185 mg/dL, and glucose is 85 mg/dL. A mutation involving a gene encoding for which of the following is most likely present in this man's family?

A. Angiotensin
B. Apolipoprotein (Correct Answer)
C. Endothelin
D. Factor VIII

Explanation: **Explanation:** The patient presents with premature coronary artery disease (angina at age 29) despite having normal blood pressure and glucose levels. In a young patient with a strong family history of cardiovascular disease (CVD) and relatively normal total cholesterol (185 mg/dL), the most likely underlying pathology is a genetic lipid disorder, specifically **Familial Dysbetalipoproteinemia (Type III Hyperlipoproteinemia)** or **Familial Combined Hyperlipidemia**. **Why Apolipoprotein is correct:** Genetic mutations in **Apolipoproteins** (such as **Apo-E** or **Apo-B100**) or their receptors (LDL-receptor) lead to impaired clearance of chylomicron remnants and IDL. Specifically, a deficiency in Apo-E prevents the liver from recognizing and clearing these atherogenic particles. This results in premature atherosclerosis and myocardial infarction even when total cholesterol levels do not appear severely elevated. **Why the other options are incorrect:** * **Angiotensin & Endothelin:** These are potent vasoconstrictors. While they play roles in hypertension and endothelial dysfunction, mutations in these genes are not classic causes of premature, familial ischemic heart disease in the absence of hypertension. * **Factor VIII:** Deficiency of Factor VIII causes Hemophilia A, a bleeding disorder. It does not predispose patients to premature atherosclerosis or angina; if anything, severe coagulation deficiencies are somewhat protective against arterial thrombosis. **NEET-PG High-Yield Pearls:** * **Type III Hyperlipoproteinemia:** Associated with **Apo-E2** homozygosity. Look for "Palmar Xanthomas" in clinical stems. * **Familial Hypercholesterolemia (Type IIa):** Caused by **LDL receptor** mutations or **Apo-B100** defects; characterized by extremely high LDL and tendon xanthomas. * **Rule of Thumb:** Any young patient (<40 years) with angina and a positive family history, in the absence of metabolic syndrome, should be evaluated for genetic apolipoprotein defects.

Question 309: An albino girl is married to a normal boy. What are the chances of their having an affected child and what are the chances of their children being carriers?

A. None affected, all carriers (Correct Answer)
B. All normal
C. 50% carriers
D. 50% affected, 50% carriers

Explanation: ### Explanation **1. Understanding the Correct Answer (Option A)** Albinism (specifically Oculocutaneous Albinism) is a classic **Autosomal Recessive (AR)** disorder. For an individual to be affected, they must possess two mutant alleles (genotype **aa**). A "normal" individual, unless specified as a carrier, is assumed to be homozygous dominant (genotype **AA**) in medical genetics problems. * **The Cross:** Mother (aa) x Father (AA) * **Punnett Square:** All offspring will have the genotype **Aa**. * **Phenotype:** Since they possess one dominant allele (A), none will manifest the disease (None affected). * **Genotype:** Since they all carry one recessive allele (a), 100% will be obligate heterozygotes (All carriers). **2. Analysis of Incorrect Options** * **Option B (All normal):** While phenotypically true, it is incomplete as it ignores the carrier status, which is a critical genetic implication. * **Option C (50% carriers):** This would only occur if the father was a carrier (Aa) and the mother was homozygous normal (AA). * **Option D (50% affected, 50% carriers):** This is the result of a **Test Cross** (aa x Aa). It only occurs if the "normal" father was actually a carrier (pseudodominance). **3. NEET-PG Clinical Pearls** * **Biochemical Defect:** Most commonly due to a deficiency of the enzyme **Tyrosinase**, which converts Tyrosine to DOPA and Melanin. * **Inheritance Pattern:** Always assume Autosomal Recessive for Albinism unless Ocular Albinism (X-linked) is specifically mentioned. * **Key Association:** Albinism is a feature of **Chédiak-Higashi syndrome** (look for giant cytoplasmic granules in neutrophils and recurrent infections). * **Risk Calculation:** In AR disorders, if both parents are carriers (Aa x Aa), the risk of an affected child is 25%, and the risk of a healthy child being a carrier is 2/3 (66%).

Question 310: The hallmarks of type Ia glycogen storage disease are all of the following except?

A. Hypoglycemia
B. Metabolic alkalosis (Correct Answer)
C. Hyperuricemia
D. Hyperlipidemia

Explanation: **Explanation:** **Type Ia Glycogen Storage Disease (Von Gierke Disease)** is caused by a deficiency of **Glucose-6-Phosphatase**, the enzyme responsible for converting Glucose-6-Phosphate into free glucose. This enzyme is crucial for both glycogenolysis and gluconeogenesis. **Why Metabolic Alkalosis is the correct answer:** In Von Gierke disease, the inability to produce glucose leads to the shunting of Glucose-6-Phosphate into the glycolytic pathway, resulting in excessive production of pyruvate and subsequently **lactic acid**. This leads to **Metabolic Acidosis** (specifically a high anion gap metabolic acidosis), not metabolic alkalosis. **Why the other options are incorrect (Hallmarks of GSD Ia):** * **Hypoglycemia (A):** Severe fasting hypoglycemia occurs because the liver cannot release free glucose into the blood from glycogen stores or via gluconeogenesis. * **Hyperuricemia (C):** Increased G-6-P levels divert into the Pentose Phosphate Pathway, increasing ribose-5-phosphate and purine synthesis. Additionally, lactic acid competes with uric acid for excretion in the kidneys, leading to gouty arthritis. * **Hyperlipidemia (D):** Low insulin and high glucagon levels trigger lipolysis. The liver also increases VLDL synthesis from excess acetyl-CoA, leading to hypertriglyceridemia and "doll-like" facies. **Clinical Pearls for NEET-PG:** * **Key Enzyme:** Glucose-6-Phosphatase (Type Ia); G-6-P Translocase (Type Ib). * **Type Ib specific:** Presents with the same features plus **neutropenia** and recurrent infections. * **Diagnostic Clue:** Hepatomegaly (due to glycogen and fat accumulation) with **no** splenomegaly. * **Treatment:** Frequent oral cornstarch (slow-release glucose) and avoidance of fructose/galactose.

Practice by Chapter

Single Gene Disorders

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Biochemical Diagnosis of Genetic Disorders

Practice Questions

Inborn Errors of Metabolism

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Lysosomal Storage Diseases

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Glycogen Storage Diseases

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Disorders of Lipoprotein Metabolism

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Disorders of Purine and Pyrimidine Metabolism

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Hemoglobinopathies

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Porphyrias

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Biochemical Markers for Disease Diagnosis

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Newborn Screening for Genetic Disorders

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Enzyme Replacement Therapy

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