Genetic Disorders and Biochemical Pathology — MCQs

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

Question 461: A child presents with seizures and developmental delay, and is diagnosed with phenylketonuria. What is the initial line of treatment for this child?

A. Dietary supply of deficient protein
B. Restriction of the dietary substrate of the deficient enzyme (Correct Answer)
C. Supplementation of enzyme products
D. Dietary supply of the deficient proenzyme

Explanation: **Explanation:** **Phenylketonuria (PKU)** is an autosomal recessive disorder caused by a deficiency of the enzyme **Phenylalanine Hydroxylase (PAH)**. This enzyme normally converts the essential amino acid Phenylalanine into Tyrosine. **1. Why Option B is Correct:** In PKU, the metabolic block leads to the toxic accumulation of **Phenylalanine** and its metabolites (like phenylpyruvate) in the blood and brain, causing seizures and intellectual disability. The primary management strategy is to **restrict the dietary intake of Phenylalanine** (the substrate) to prevent this toxic buildup. This is achieved through a specialized low-protein diet and phenylalanine-free medical formulas. **2. Why Other Options are Incorrect:** * **Option A:** Providing the "deficient protein" (PAH enzyme) orally is ineffective because enzymes are proteins that would be digested in the gastrointestinal tract before reaching the liver. * **Option C:** While Tyrosine becomes a "conditionally essential" amino acid in PKU, simply supplementing it without restricting Phenylalanine will not prevent neurotoxicity. * **Option D:** Proenzymes are inactive precursors; supplying them does not bypass the genetic defect in the PAH gene. **3. Clinical Pearls for NEET-PG:** * **Classic Presentation:** "Mousy" or "Musty" body odor, fair skin/blue eyes (due to decreased melanin), and microcephaly. * **Diagnosis:** Guthrie test (bacterial inhibition assay) or Tandem Mass Spectrometry (TMS) for newborn screening. * **Target Levels:** Phenylalanine levels should be maintained between **2–6 mg/dL**. * **Maternal PKU:** If a pregnant woman with PKU does not maintain a strict diet, the high phenylalanine levels act as a **teratogen**, causing fetal microcephaly and congenital heart defects.

Question 462: Which of the following is a classic example of a missense mutation?

A. Thalassemia
B. Sickle cell disease (Correct Answer)
C. Sideroblastic anemia
D. Hemochromatosis

Explanation: **Explanation:** **Sickle Cell Disease (SCD)** is the classic prototype of a **missense mutation**. In SCD, a single nucleotide substitution occurs in the $\beta$-globin gene on chromosome 11. Specifically, adenine is replaced by thymine (**GAG $\rightarrow$ GTG**). This results in the substitution of the amino acid **Glutamic acid** (polar/hydrophilic) with **Valine** (non-polar/hydrophobic) at the **6th position** of the $\beta$-globin chain. This single amino acid change causes the hemoglobin (HbS) to polymerize under deoxygenated conditions, leading to the characteristic "sickling" of red blood cells. **Analysis of Incorrect Options:** * **Thalassemia:** These are typically caused by **quantitative** defects. $\beta$-thalassemia is most commonly due to **splicing mutations** or point mutations leading to premature stop codons (nonsense mutations), while $\alpha$-thalassemia is usually due to **gene deletions**. * **Sideroblastic Anemia:** This is a group of disorders characterized by impaired heme synthesis. The most common hereditary form is X-linked, caused by a mutation in the **ALAS2 gene**, but it is not defined as a classic missense model in the same way SCD is. * **Hemochromatosis:** While the most common mutation (C282Y) in the HFE gene is a missense mutation, it is not the "classic" textbook example used to teach the concept of point mutations in medical biochemistry. **High-Yield Clinical Pearls for NEET-PG:** * **Point Mutation Types:** * **Silent:** Same amino acid. * **Missense:** Different amino acid (e.g., Sickle Cell). * **Nonsense:** Results in a premature STOP codon (UAG, UAA, UGA). * **HbS Electrophoresis:** On alkaline electrophoresis, HbS moves **slower** than HbA toward the anode because valine is less negatively charged than glutamic acid. * **Glutamic acid $\rightarrow$ Lysine** at the 6th position results in **HbC disease**.

Question 463: The FeCl3 test for urine in maple syrup urine disease gives which color?

A. Green
B. Blue (Correct Answer)
C. Black
D. Red

Explanation: **Explanation:** The **Ferric Chloride (FeCl₃) test** is a classic biochemical screening tool used to detect specific metabolites (usually phenols, enols, or keto-acids) in the urine. **1. Why Blue is Correct:** In **Maple Syrup Urine Disease (MSUD)**, there is a deficiency of the **Branched-Chain Alpha-Keto Acid Dehydrogenase (BCKAD)** complex. This leads to the accumulation of branched-chain amino acids (Leucine, Isoleucine, Valine) and their corresponding **alpha-keto acids**. Specifically, the presence of **alpha-ketoisovaleric acid** reacts with ferric chloride to produce a characteristic **navy blue** or bluish-gray color. **2. Analysis of Incorrect Options:** * **Green (Option A):** This is the classic result for **Phenylketonuria (PKU)** due to the presence of phenylpyruvic acid. It is a very high-yield distinction for NEET-PG. * **Black (Option C):** A "transient blue-green" turning into a **persistent black** color is seen in **Alkaptonuria** (due to homogentisic acid). * **Red/Purple (Option D):** A purple or Bordeaux-red color is seen in **Salicylate poisoning** or the presence of **Acetoacetate** (Ketone bodies). **3. High-Yield Clinical Pearls for MSUD:** * **Defective Enzyme:** BCKAD (requires Thiamine/B1 as a cofactor). * **Odor:** Urine smells like burnt sugar or maple syrup due to **S-isoleucine**. * **Diagnosis:** Elevated levels of Leucine, Isoleucine, and Valine in plasma; presence of **Alloisoleucine** (pathognomonic). * **Management:** Dietary restriction of BCAA and, in some cases, high-dose Thiamine supplementation.

Question 464: Which type of mutation occurs in sickle cell anemia?

A. Crossover mutation
B. Frameshift
C. Deletion
D. Point mutation (Correct Answer)

Explanation: **Explanation:** **Sickle Cell Anemia** is a classic example of a **Point Mutation**, specifically a **missense mutation**. It involves a single nucleotide substitution in the $\beta$-globin gene on chromosome 11. 1. **Why Point Mutation is Correct:** The molecular basis involves the substitution of **Adenine (A) with Thymine (T)** in the GAG codon (GAG $\rightarrow$ GTG). This results in the replacement of **Glutamic acid** (a polar, negatively charged amino acid) with **Valine** (a non-polar, hydrophobic amino acid) at the **6th position** of the $\beta$-globin chain. This single change causes hemoglobin (HbS) to polymerize under deoxygenated conditions, leading to the characteristic "sickling" of RBCs. 2. **Why Other Options are Incorrect:** * **Frameshift Mutation:** Occurs when nucleotides are inserted or deleted in numbers not divisible by three, altering the entire reading frame (e.g., Tay-Sachs disease). Sickle cell does not change the reading frame. * **Deletion:** Involves the loss of genetic material. While $\alpha$-thalassemia often results from gene deletions, sickle cell is a substitution, not a loss. * **Crossover Mutation:** Refers to unequal crossing over during meiosis (e.g., Hb Lepore). This is a large-scale structural rearrangement, not a single base change. **High-Yield NEET-PG Pearls:** * **Inheritance:** Autosomal Recessive. * **Electrophoresis:** HbS moves **slower** than HbA toward the anode (+) because valine is neutral, whereas glutamic acid is negatively charged. * **Protective Effect:** Heterozygotes (Sickle cell trait) show resistance to *Plasmodium falciparum* malaria. * **Metabolic Trigger:** Acidosis, dehydration, and hypoxia precipitate sickling crises.

Question 465: What syndrome is associated with the deficiency of Dermatan sulfate, heparan sulfate, chondroitin 4-sulfate, and chondroitin 6-sulfate?

A. Hunter syndrome
B. Morquio syndrome B
C. Sly syndrome (Correct Answer)
D. Hurler syndrome

Explanation: ### Explanation **Correct Answer: C. Sly syndrome** **Underlying Medical Concept:** Sly syndrome (Mucopolysaccharidosis type VII) is caused by a deficiency of the enzyme **β-glucuronidase**. This specific enzyme is required for the degradation of glucuronic acid residues found in multiple glycosaminoglycans (GAGs). Because glucuronic acid is a common constituent of **Dermatan sulfate, Heparan sulfate, and Chondroitin 4/6-sulfates**, a deficiency in β-glucuronidase leads to the systemic accumulation of all four substances. This distinguishes it from other MPS types which typically involve only one or two GAGs. **Analysis of Incorrect Options:** * **A. Hunter syndrome (MPS II):** Caused by a deficiency of **Iduronate sulfatase**. It leads to the accumulation of **Dermatan sulfate and Heparan sulfate** only. (Key NEET-PG fact: It is X-linked recessive and lacks corneal clouding). * **B. Morquio syndrome B (MPS IVB):** Caused by a deficiency of **β-galactosidase**. It primarily results in the accumulation of **Keratan sulfate**. * **D. Hurler syndrome (MPS IH):** Caused by a deficiency of **α-L-iduronidase**. Like Hunter syndrome, it leads to the accumulation of **Dermatan sulfate and Heparan sulfate**, but presents with more severe features and corneal clouding. **High-Yield Clinical Pearls for NEET-PG:** * **Sly Syndrome Unique Feature:** It is the only MPS that can present as **Hydrops fetalis** in utero. * **GAG Triad:** If a question mentions the accumulation of **Chondroitin sulfates** alongside Dermatan and Heparan, always think of **Sly Syndrome**. * **Enzyme Mnemonic:** "Sly (7) Glucuronidase" — MPS **VII** is **β-glucuronidase** deficiency.

Question 466: In Phenylketonuria, what is the main aim of first-line therapy?

A. Replacement of the defective enzyme
B. Replacement of the deficient product
C. Limiting the substrate for the deficient enzyme (Correct Answer)
D. Giving the missing amino acid by diet

Explanation: ### Explanation **1. Why Option C is Correct:** Phenylketonuria (PKU) is most commonly caused by a deficiency of the enzyme **Phenylalanine Hydroxylase (PAH)**, which converts the essential amino acid **Phenylalanine** into **Tyrosine**. In its absence, Phenylalanine accumulates to toxic levels in the blood and brain, leading to severe intellectual disability and seizures. The primary therapeutic strategy is **dietary restriction of Phenylalanine**. By limiting the substrate (Phenylalanine intake), we prevent its toxic accumulation, thereby bypassing the metabolic block. **2. Why the Other Options are Incorrect:** * **Option A:** Enzyme Replacement Therapy (ERT) exists (e.g., Pegvaliase), but it is not the "first-line" therapy. It is generally reserved for adults who cannot maintain metabolic control through diet alone. * **Option B:** While Tyrosine becomes a "conditionally essential" amino acid in PKU, simply replacing the product (Tyrosine) does not solve the primary problem of Phenylalanine neurotoxicity. * **Option C vs D:** Option D is a distractor. Phenylalanine is an essential amino acid; we do not "give" it because it is missing—we strictly "limit" it because it is in excess. **3. High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** Autosomal Recessive. * **Cofactor:** A subset of PKU is caused by a deficiency of **Tetrahydrobiopterin (BH4)**. * **Clinical Features:** "Mousy" or "Musty" body odor (due to phenylacetic acid), hypopigmentation (low melanin), and intellectual disability. * **Diagnosis:** Guthrie Test (bacterial inhibition assay) or Tandem Mass Spectrometry (TMS) for newborn screening. * **Maternal PKU:** If a pregnant woman with PKU doesn't maintain a strict diet, the high phenylalanine levels act as a **teratogen**, causing microcephaly and congenital heart defects in the fetus.

Question 467: Which glycogen storage disease does not affect muscles?

A. Type I (Correct Answer)
B. Type II
C. Type III
D. Type V

Explanation: **Explanation:** The correct answer is **Type I (von Gierke disease)**. **1. Why Type I is correct:** Type I Glycogen Storage Disease (GSD) is caused by a deficiency of **Glucose-6-Phosphatase**. This enzyme is primarily expressed in the **liver and kidneys**, where it is responsible for the final step of gluconeogenesis and glycogenolysis (converting Glucose-6-Phosphate to free Glucose). Since skeletal muscle lacks this enzyme even under physiological conditions (it lacks the ability to release glucose into the blood), the pathology of Type I is confined to the liver and kidneys. Consequently, patients present with severe hypoglycemia and hepatomegaly, but **no muscle symptoms** (no weakness or cramping). **2. Why the other options are incorrect:** * **Type II (Pompe disease):** Caused by a deficiency in **Lysosomal α-1,4-glucosidase (Acid Maltase)**. This enzyme is present in all tissues; its deficiency leads to glycogen accumulation in lysosomes, severely affecting cardiac and skeletal muscle (leading to cardiomyopathy and hypotonia). * **Type III (Cori disease):** Caused by a deficiency of the **Debranching enzyme**. Unlike Type I, this enzyme is expressed in both the liver and muscles. Therefore, patients present with both hepatomegaly and **skeletal myopathy**. * **Type V (McArdle disease):** Caused by a deficiency of **Skeletal Muscle Glycogen Phosphorylase (Myophosphorylase)**. This is a purely **muscle-specific** GSD, presenting with exercise-induced cramps and myoglobinuria. **High-Yield Clinical Pearls for NEET-PG:** * **Type I (von Gierke):** Look for "Doll-like facies," hyperuricemia (gout), and lactic acidosis. * **Type II (Pompe):** "Pompe trashes the Pump" (Heart). It is the only GSD that is also a Lysosomal Storage Disease. * **Type V (McArdle):** Characterized by a flat venous lactate curve during the ischemic exercise test. * **Mnemonic:** Types I, III, IV, and VI primarily affect the liver; Types II, III, V, and VII affect the muscles. Note that **Type III** affects both.

Question 468: An 8-day-old child presents with yellow sclera, whitish stool, and turmeric-colored urine on the 3rd day of septicemia treated with broad-spectrum antibiotics. What is the likely diagnosis?

A. Galactosidase deficiency
B. Ammonia toxicity
C. Galactose-1-phosphate uridyltransferase deficiency (Correct Answer)
D. All of the above

Explanation: **Explanation:** The clinical presentation of jaundice (yellow sclera), acholic stools (whitish stool), and dark urine (turmeric-colored) in a neonate, specifically following the onset of **septicemia**, is a classic hallmark of **Classic Galactosemia** (Galactose-1-phosphate uridyltransferase or GALT deficiency). **Why Option C is Correct:** In GALT deficiency, the body cannot metabolize galactose (found in breast milk/formula). This leads to the accumulation of **Galactose-1-phosphate**, which is hepatotoxic. The resulting liver injury causes conjugated hyperbilirubinemia (leading to dark urine and pale stools). A high-yield association for NEET-PG is that these infants are predisposed to **E. coli sepsis**, which often precedes or coincides with the clinical diagnosis of jaundice. **Why Other Options are Incorrect:** * **Option A (Galactosidase deficiency):** This refers to conditions like Fabry disease (alpha-galactosidase A) or GM1 gangliosidosis (beta-galactosidase). These are lysosomal storage disorders that typically present later in infancy or childhood with organomegaly or neurological regression, not acute neonatal jaundice and sepsis. * **Option B (Ammonia toxicity):** While GALT deficiency can lead to secondary liver failure and hyperammonemia, ammonia toxicity itself (e.g., Urea Cycle Disorders) typically presents with lethargy, vomiting, and seizures without the prominent conjugated jaundice and acholic stools seen here. **Clinical Pearls for NEET-PG:** * **Enzyme Defect:** GALT (Chromosome 9p). * **Classic Triad:** Cataracts ("Oil drop"), Liver failure (Jaundice/Hepatomegaly), and Intellectual disability. * **Sepsis Link:** Increased susceptibility to **Gram-negative organisms**, specifically *E. coli*. * **Screening:** Reducing substances in urine (Clinitest positive) but negative for glucose (Dipstick). * **Management:** Immediate cessation of lactose/galactose-containing milk; switch to soy-based formula.

Question 469: What is the most appropriate treatment for Homocystinuria?

A. Betaine
B. Folic acid
C. Vitamin C
D. Vitamin B6 (Correct Answer)

Explanation: **Explanation:** **Homocystinuria** is most commonly caused by a deficiency of the enzyme **Cystathionine β-synthase (CBS)**. This enzyme facilitates the conversion of homocysteine to cystathionine. Crucially, CBS requires **Pyridoxine (Vitamin B6)** as a mandatory cofactor. 1. **Why Vitamin B6 is correct:** Approximately 50% of patients with Homocystinuria are "B6-responsive." High doses of Vitamin B6 can stabilize the mutated CBS enzyme, enhancing its residual activity and effectively lowering toxic homocysteine levels. It is considered the first-line pharmacological intervention. 2. **Why other options are incorrect:** * **Betaine (A):** While used as an adjunct treatment, it acts by providing methyl groups to convert homocysteine back to methionine (remethylation pathway). It is typically reserved for B6-non-responsive patients. * **Folic acid (B):** Folate and Vitamin B12 are essential cofactors for the remethylation of homocysteine, but they are supportive therapies rather than the primary corrective treatment for the classic CBS deficiency. * **Vitamin C (C):** Has no direct role in the metabolic pathway of sulfur-containing amino acids or the treatment of Homocystinuria. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Triad:** Intellectual disability, Ectopia lentis (downward dislocation of lens), and Thromboembolic events (major cause of mortality). * **Marfanoid Habitus:** Patients often present with long limbs and chest deformities, but unlike Marfan syndrome (upward lens dislocation), Homocystinuria features downward lens dislocation and autosomal recessive inheritance. * **Diagnosis:** Elevated homocysteine in urine (Cyanide-nitroprusside test) and blood. * **Dietary Management:** Low methionine diet and Cysteine supplementation (as Cysteine becomes an essential amino acid).

Question 470: Which of the following findings is associated with alkaptonuria?

A. Ochronosis (Correct Answer)
B. Red urine
C. Anuria
D. Myositis

Explanation: **Explanation:** **Alkaptonuria** is an autosomal recessive disorder caused by a deficiency of the enzyme **homogentisate 1,2-dioxygenase**. This enzyme is essential in the catabolic pathway of phenylalanine and tyrosine. Its deficiency leads to the accumulation of **homogentisic acid (HGA)** in the body. 1. **Why Ochronosis is Correct:** Excess homogentisic acid is oxidized and polymerized into a melanin-like pigment. This pigment deposits in connective tissues, cartilages (like the pinna of the ear and nose), and joints. This bluish-black discoloration of connective tissue is termed **ochronosis**. Long-term deposition leads to ochronotic arthritis, typically affecting large weight-bearing joints and the spine. 2. **Why Other Options are Incorrect:** * **Red urine:** This is characteristic of Porphyrias or Hematuria. In Alkaptonuria, the urine turns **black** upon standing or alkalinization due to the oxidation of HGA. * **Anuria:** This refers to the failure of kidneys to produce urine, seen in acute renal failure, not in amino acid metabolism disorders. * **Myositis:** This is an inflammatory muscle condition (e.g., Dermatomyositis) and is not a feature of Alkaptonuria. **High-Yield Clinical Pearls for NEET-PG:** * **Triad of Alkaptonuria:** Homogentisic aciduria (urine turns black on standing), Ochronosis, and Arthritis. * **Diagnostic Test:** Ferric chloride test (gives a transient deep blue color) and Silver nitrate test. * **Radiological Sign:** "Bamboo spine" appearance due to calcification of intervertebral discs (mimicking Ankylosing Spondylitis). * **Management:** Dietary restriction of Tyrosine and Phenylalanine; **Nitisinone** is the drug of choice as it inhibits the enzyme 4-hydroxyphenylpyruvate dioxygenase, preventing HGA formation.

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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