Genetic Disorders and Biochemical Pathology — MCQs

A 15-year-old boy with Albright hereditary osteodystrophy (AHO) presents with severe muscle cramps and convulsions. The child has a history of mental retardation. Laboratory studies reveal hypocalcemia and elevated blood levels of parathyroid hormone (PTH). Which of the following distinguishes this patient's endocrinopathy from the hypoparathyroidism seen in DiGeorge syndrome?

Q416Medium

All of the following are true about pyruvate dehydrogenase deficiency, EXCEPT?

Q417Easy

Deficiency of acid lipase leads to which of the following genetic disorders?

Q418Medium

An albino girl gets married to a normal boy. What are the expected genotypes of their offspring?

Q419Easy

All of the following are hepatic porphyrias, EXCEPT?

Q420Easy

Brain damage in phenylketonuria is due to the accumulation of which substance?

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

Question 411: Sickle cell anaemia is due to:

A. Addition
B. Deletion
C. Point mutation (Correct Answer)
D. None

Explanation: **Explanation:** **Sickle Cell Anaemia (SCA)** is a classic example of a **Point Mutation**, specifically a **missense mutation**. It occurs due to a single nucleotide substitution in the **$\beta$-globin gene** located on **Chromosome 11**. 1. **Why Point Mutation is Correct:** The molecular defect involves a transversion where Adenine is replaced by Thymine (**GAG $\rightarrow$ GTG**) at the **6th codon** of the $\beta$-globin chain. This results in the substitution of the amino acid **Glutamic acid** (polar/hydrophilic) with **Valine** (non-polar/hydrophobic). This single change creates a "sticky patch" on the hemoglobin molecule (HbS), leading to polymerization under deoxygenated conditions, causing the characteristic "sickle" shape of RBCs. 2. **Why Other Options are Incorrect:** * **Addition/Deletion:** These are "Frameshift mutations" (if not in multiples of three). They alter the entire reading frame of the DNA sequence from the point of mutation, leading to completely non-functional proteins. Examples include certain types of $\beta$-Thalassemia, but not SCA. 3. **High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** Autosomal Recessive. * **Electrophoresis:** On alkaline electrophoresis (pH 8.6), HbS moves **slower** than HbA towards the anode because Valine is neutral, whereas Glutamic acid is negatively charged. * **Protective Effect:** Heterozygotes (Sickle cell trait) show resistance against *Plasmodium falciparum* malaria. * **Diagnosis:** Solubility test (screening) and Hb Electrophoresis/HPLC (confirmatory). * **Metabolic Trigger:** Acidosis, hypoxia, and dehydration promote sickling.

Question 412: Thalassemia occurs due to which types of mutations?

A. Missense mutations
B. Splicing mutations
C. Transition mutations (Correct Answer)
D. Frame-shift mutations

Explanation: **Explanation:** Thalassemia is a quantitative hemoglobinopathy characterized by the reduced synthesis of alpha or beta-globin chains. The molecular basis of Thalassemia is highly heterogeneous, involving various types of mutations. **Why Transition Mutations are Correct:** Point mutations are the most common cause of **Beta-Thalassemia**. Among point mutations, **Transitions** (replacement of a purine by a purine or a pyrimidine by a pyrimidine) are statistically more frequent than transversions. These mutations often occur in the promoter region, splice sites, or coding sequences, leading to reduced or absent mRNA production. Specifically, many common Beta-Thalassemia alleles in the Indian subcontinent involve C→T or G→A transitions. **Analysis of Other Options:** * **A. Missense mutations:** These typically result in **qualitative** defects (e.g., Sickle Cell Anemia, where Glutamic acid is replaced by Valine), rather than the quantitative reduction seen in Thalassemia. * **B. Splicing mutations:** While splicing defects are a major cause of Beta-Thalassemia, the question asks for the *type* of nucleotide substitution. Splicing defects are often the *result* of a transition mutation at the splice junction. * **D. Frame-shift mutations:** These occur due to deletions or insertions. While they do cause Thalassemia (especially $\beta^0$), they are less common than point mutations (transitions) in the global population of Thalassemia patients. **High-Yield Clinical Pearls for NEET-PG:** * **Alpha-Thalassemia:** Most commonly caused by **large gene deletions** (e.g., Southeast Asian/Mediterranean deletions). * **Beta-Thalassemia:** Most commonly caused by **point mutations** (Transitions). * **Hallmark:** Microcytic hypochromic anemia with a normal or elevated serum iron/ferritin level (differentiating it from Iron Deficiency Anemia). * **Diagnosis:** Hb Electrophoresis shows elevated **HbA2 (>3.5%)** in Beta-Thalassemia minor.

Question 413: Which of the following statements is TRUE regarding a 3-week-old child with phenylketonuria, EXCEPT?

A. Provocative protein meal tests help in the diagnosis.
B. Tyrosine becomes an essential amino acid in the diet.
C. Serum phenylalanine is increased and urinary phenyl pyruvate level is elevated.
D. Phenylalanine should be completely stopped in the diet. (Correct Answer)

Explanation: **Explanation:** **Why Option D is the correct answer (The Exception):** Phenylalanine is an **essential amino acid** required for normal growth, protein synthesis, and tissue repair. In Phenylketonuria (PKU), the goal is to **restrict** phenylalanine intake to maintain blood levels within a safe range (2–6 mg/dL), not to eliminate it entirely. Complete removal from the diet would lead to severe protein deficiency, growth failure, and even death. **Analysis of other options:** * **Option A:** Provocative protein meal tests (or phenylalanine loading tests) can be used to differentiate between classical PKU and transient hyperphenylalaninemia by observing the rate of clearance. * **Option B:** In PKU, the enzyme **Phenylalanine Hydroxylase (PAH)** is deficient. Since tyrosine is normally synthesized from phenylalanine via this enzyme, it becomes a **conditionally essential amino acid** that must be supplemented in the diet. * **Option C:** Due to the metabolic block, phenylalanine accumulates in the blood (Hyperphenylalaninemia). It is then diverted to alternative pathways, forming phenylketones like **phenylpyruvate**, which is excreted in the urine (giving it a characteristic "mousy" odor). **High-Yield Clinical Pearls for NEET-PG:** * **Enzyme Deficiency:** Most commonly Phenylalanine Hydroxylase; rarely **Dihydropteridine reductase** (malignant PKU). * **Cofactor:** Tetrahydrobiopterin (**BH4**). * **Clinical Features:** Intellectual disability, seizures, "mousy" body odor, and hypopigmentation (due to decreased melanin synthesis from tyrosine). * **Screening:** Guthrie Test (bacterial inhibition assay) or Tandem Mass Spectrometry. * **Management:** "Diet for life" (low phenylalanine) and avoiding **Aspartame** (which contains phenylalanine).

Question 414: G6PD helps in maintaining the integrity of RBC by:

A. Controlling oxidative stress on RBC (Correct Answer)
B. Controlling reduction stress on RBC
C. Maintaining flexibility of cell membrane
D. Component of electron transport chain

Explanation: **Explanation:** **Glucose-6-Phosphate Dehydrogenase (G6PD)** is the rate-limiting enzyme of the **Hexose Monophosphate (HMP) Shunt**. Its primary function in Red Blood Cells (RBCs) is the production of **NADPH**. 1. **Why Option A is Correct:** RBCs are constantly exposed to Reactive Oxygen Species (ROS) like hydrogen peroxide ($H_2O_2$). To neutralize these, the cell uses **Reduced Glutathione**. During this process, glutathione becomes oxidized. To regenerate reduced glutathione, the enzyme **Glutathione Reductase** requires **NADPH** (produced by G6PD) as a cofactor. Without G6PD, NADPH levels drop, oxidized glutathione accumulates, and oxidative stress causes hemoglobin to denature, forming **Heinz bodies** and leading to hemolysis. 2. **Why Other Options are Incorrect:** * **Option B:** "Reduction stress" is not a physiological challenge for RBCs; the threat is always oxidative damage from free radicals. * **Option C:** While G6PD deficiency eventually leads to membrane damage (via Heinz body removal by splenic macrophages), membrane flexibility is primarily maintained by cytoskeletal proteins like **Spectrin and Ankyrin** (defective in Hereditary Spherocytosis). * **Option D:** The Electron Transport Chain (ETC) is located in the mitochondria. Mature RBCs lack mitochondria and rely solely on glycolysis for energy. **High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** G6PD deficiency is an **X-linked Recessive** disorder. * **Triggers:** Hemolysis is triggered by **Fava beans**, infections, or drugs (e.g., **Primaquine**, Sulfa drugs, Nitrofurantoin). * **Morphology:** Look for **Heinz Bodies** (denatured Hb) and **Bite Cells** (deformed RBCs after splenic macrophages "bite" out the Heinz bodies). * **Protection:** G6PD deficiency offers a selective advantage against **Falciparum malaria**.

Question 415: A 15-year-old boy with Albright hereditary osteodystrophy (AHO) presents with severe muscle cramps and convulsions. The child has a history of mental retardation. Laboratory studies reveal hypocalcemia and elevated blood levels of parathyroid hormone (PTH). Which of the following distinguishes this patient's endocrinopathy from the hypoparathyroidism seen in DiGeorge syndrome?

A. Abnormalities in cardiac conduction and contractility
B. Accelerated degradation of PTH
C. Decreased neuromuscular excitability
D. End-organ unresponsiveness to PTH (Correct Answer)

Explanation: ### Explanation **1. Why the Correct Answer is Right:** The patient presents with **Pseudohypoparathyroidism (PHP) Type 1A**, which is the biochemical hallmark of **Albright Hereditary Osteodystrophy (AHO)**. The underlying defect is a mutation in the **GNAS1 gene**, which encodes the alpha subunit of the **stimulatory G-protein (Gsα)**. In this condition, the kidneys and bones are **unresponsive to Parathyroid Hormone (PTH)**. Because the receptors cannot trigger the downstream signaling (cAMP), the body perceives a deficiency of PTH despite high circulating levels. This leads to the classic biochemical triad: **Hypocalcemia, Hyperphosphatemia, and Elevated PTH**. In contrast, DiGeorge syndrome involves true hypoparathyroidism (low PTH) due to thymic and parathyroid aplasia. **2. Analysis of Incorrect Options:** * **A. Abnormalities in cardiac conduction:** Both PHP and DiGeorge syndrome cause hypocalcemia, which leads to a prolonged QT interval. This does not distinguish between the two. * **B. Accelerated degradation of PTH:** In PHP, PTH is produced normally but fails to act. There is no pathology related to the degradation rate of the hormone. * **C. Decreased neuromuscular excitability:** Hypocalcemia (in both conditions) causes **increased** neuromuscular excitability, leading to tetany, Chvostek’s sign, and Trousseau’s sign. **3. High-Yield Clinical Pearls for NEET-PG:** * **AHO Phenotype:** Short stature, round face (moon facies), obesity, developmental delay, and **shortened 4th/5th metacarpals** (Archibald’s sign). * **Pseudopseudohypoparathyroidism (PPHP):** Occurs when the same GNAS mutation is inherited paternally. Patients have the AHO phenotype but **normal** calcium and PTH levels (due to genomic imprinting). * **Diagnostic Test:** Administration of exogenous PTH fails to increase urinary cAMP or phosphate in PHP Type 1A (Ellsworth-Howard test).

Question 416: All of the following are true about pyruvate dehydrogenase deficiency, EXCEPT?

A. Mutation in the E1 component of PDH is the most common.
B. Decrease in blood lactate. (Correct Answer)
C. Ketogenic diet rich in leucine is given as a treatment.
D. Rise in pyruvate.

Explanation: **Explanation:** **Pyruvate Dehydrogenase (PDH) Deficiency** is a common cause of congenital lactic acidosis. The PDH complex is responsible for converting Pyruvate into Acetyl-CoA, linking glycolysis to the TCA cycle. **1. Why "Decrease in blood lactate" is the correct answer (The Exception):** When the PDH complex is deficient, Pyruvate cannot be converted into Acetyl-CoA. This leads to a "backlog" of pyruvate. To regenerate NAD+ for glycolysis to continue, the body shunts the excess pyruvate into the **Lactic Acid pathway** via Lactate Dehydrogenase. This results in **Hyperlactatemia (increased blood lactate)** and lactic acidosis, not a decrease. **2. Analysis of Incorrect Options:** * **Option A:** The PDH complex consists of three enzymes (E1, E2, E3). Mutation in the **E1-alpha subunit** (located on the X chromosome) is indeed the **most common** cause of PDH deficiency. * **Option C:** A **Ketogenic diet** (high fat, low carbohydrate) is the treatment of choice. It bypasses PDH by providing Acetyl-CoA directly from fatty acid oxidation. **Leucine and Lysine** are exclusively ketogenic amino acids that do not contribute to pyruvate formation, making them ideal for these patients. * **Option D:** As the conversion of pyruvate to Acetyl-CoA is blocked, there is a significant **rise in pyruvate** levels in the blood. **Clinical Pearls for NEET-PG:** * **Presentation:** Infantile-onset hypotonia, psychomotor retardation, and seizures. * **Biochemical Hallmark:** Elevated Lactate, Pyruvate, and Alanine (via transamination). * **Inheritance:** Most common form (E1 deficiency) is **X-linked Dominant**, though other subunits follow Autosomal Recessive patterns. * **Management:** Avoid glucose/carbohydrates; supplement with **Thiamine** (Vitamin B1) as it is a cofactor for the E1 subunit.

Question 417: Deficiency of acid lipase leads to which of the following genetic disorders?

A. Fabry disease
B. Gaucher disease
C. Farber disease
D. Wolman disease (Correct Answer)

Explanation: **Explanation:** **Wolman Disease** is the correct answer because it is caused by a severe deficiency of **Lysosomal Acid Lipase (LAL)**. This enzyme is responsible for hydrolyzing cholesteryl esters and triglycerides within lysosomes. When deficient, these lipids accumulate massively in the liver, spleen, and adrenal glands. A hallmark clinical finding is **bilateral adrenal calcification**, which is highly characteristic and often tested in NEET-PG. **Analysis of Incorrect Options:** * **Fabry Disease:** Caused by a deficiency of **$\alpha$-galactosidase A**, leading to the accumulation of ceramide trihexoside. It is X-linked recessive and presents with angiokeratomas, peripheral neuropathy, and renal failure. * **Gaucher Disease:** The most common lysosomal storage disorder, caused by a deficiency of **$\beta$-glucocerebrosidase** (glucosylceramidase). It features "crinkled paper" appearance macrophages and hepatosplenomegaly. * **Farber Disease:** Caused by a deficiency of **ceramidase**, leading to the accumulation of ceramide. It is clinically characterized by the triad of painful joint swelling, subcutaneous nodules, and hoarseness (due to laryngeal involvement). **High-Yield Clinical Pearls for NEET-PG:** * **LAL Deficiency Spectrum:** Wolman disease is the early-onset, severe form (infantile). **Cholesteryl Ester Storage Disease (CESD)** is the milder, late-onset form caused by the same enzyme deficiency. * **Adrenal Calcification:** If a clinical vignette mentions an infant with hepatosplenomegaly and calcified adrenal glands on X-ray, the diagnosis is Wolman disease. * **Inheritance:** All options listed are Autosomal Recessive, except for Fabry disease, which is **X-linked Recessive**.

Question 418: An albino girl gets married to a normal boy. What are the expected genotypes of their offspring?

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, most commonly caused by a deficiency of the enzyme **tyrosinase**. * **The Mother:** Being an albino, her genotype must be homozygous recessive (**aa**). * **The Father:** Described as "normal" (and in the absence of information suggesting he is a carrier, we assume he is homozygous dominant, **AA**). * **The Cross:** A Punnett square between **aa** (mother) and **AA** (father) results in 100% of the offspring having the genotype **Aa**. Since they carry one dominant allele, they are phenotypically normal but are obligate genetic **carriers**. **2. Why Other Options are Incorrect** * **Option B (All normal):** While the children are phenotypically normal, this option is incomplete as it ignores their status as carriers, which is crucial in genetic counseling. * **Option C (50% carriers):** This would only occur if the father was a carrier (Aa) and the mother was normal (AA). * **Option D (50% affected, 50% carriers):** This is the expected outcome of a **Test Cross** (aa × Aa), occurring only if the "normal" father was actually a heterozygous carrier. **3. NEET-PG High-Yield Pearls** * **Biochemical Defect:** Most common cause is a mutation in the *TYR* gene leading to **Tyrosinase deficiency**, which fails to convert Tyrosine to DOPA and Melanin. * **Inheritance Pattern:** Always assume Albinism is **Autosomal Recessive** unless specified otherwise (e.g., Ocular Albinism is X-linked). * **Clinical Feature:** Increased risk of **Squamous Cell Carcinoma** due to lack of protective melanin against UV radiation. * **Key Association:** **Chédiak-Higashi syndrome** also presents with partial albinism, but includes giant lysosomal granules and immunodeficiency.

Question 419: All of the following are hepatic porphyrias, EXCEPT?

A. Variegate porphyria
B. Acute intermittent porphyria
C. Porphyria Cutanea Tarda
D. Protoporphyria (Correct Answer)

Explanation: **Explanation:** Porphyrias are metabolic disorders caused by deficiencies in the enzymes of the heme biosynthetic pathway. They are traditionally classified based on the primary site of overproduction and accumulation of porphyrin precursors into **Hepatic** and **Erythropoietic** types. **Why Protoporphyria is the correct answer:** **Protoporphyria** (specifically Erythropoietic Protoporphyria or EPP) is classified as an **Erythropoietic porphyria**. It results from a deficiency of the enzyme **Ferrochelatase**. In this condition, protoporphyrin IX accumulates primarily in the reticulocytes and bone marrow, leading to severe cutaneous photosensitivity. **Analysis of Incorrect Options:** * **Acute Intermittent Porphyria (AIP):** A classic **Hepatic** porphyria caused by Porphobilinogen deaminase deficiency. It presents with neurological symptoms (abdominal pain, psychosis) but no photosensitivity. * **Variegate Porphyria (VP):** A **Hepatic** porphyria caused by Protoporphyrinogen oxidase deficiency. It presents with both neurological symptoms and cutaneous photosensitivity. * **Porphyria Cutanea Tarda (PCT):** The most common **Hepatic** porphyria, caused by Uroporphyrinogen decarboxylase deficiency. It is characterized by chronic skin blistering and is often associated with liver damage or iron overload. **High-Yield Clinical Pearls for NEET-PG:** * **Most common porphyria:** Porphyria Cutanea Tarda (PCT). * **Most common acute porphyria:** Acute Intermittent Porphyria (AIP). * **Enzyme deficient in AIP:** PBG Deaminase (HMB Synthase). * **Photosensitivity:** Absent in AIP (because the block is before the formation of porphyrin rings). * **Urine finding in AIP:** Urine darkens on standing (due to oxidation of PBG to porphobilin).

Question 420: Brain damage in phenylketonuria is due to the accumulation of which substance?

A. Tyrosine
B. Phenylalanine (Correct Answer)
C. Tryptophan
D. None of the above

Explanation: **Explanation:** **Phenylketonuria (PKU)** is an autosomal recessive disorder caused by a deficiency of the enzyme **Phenylalanine Hydroxylase (PAH)** or its cofactor, **Tetrahydrobiopterin (BH4)**. This enzyme normally converts Phenylalanine into Tyrosine. **Why Phenylalanine is the correct answer:** The primary cause of brain damage in PKU is the **toxic accumulation of Phenylalanine** in the blood and brain. High levels of Phenylalanine saturate the Large Neutral Amino Acid Transporter (LAT1) at the blood-brain barrier. This competitively inhibits the transport of other essential amino acids (like Tyrosine and Tryptophan) into the brain, leading to: 1. **Defective Myelination:** Phenylalanine interferes with cholesterol synthesis and myelin formation. 2. **Neurotransmitter Deficiency:** Reduced brain levels of Tyrosine and Tryptophan lead to decreased synthesis of Dopamine, Norepinephrine, and Serotonin. **Why other options are incorrect:** * **Tyrosine:** In PKU, Tyrosine becomes an **essential amino acid** because it cannot be synthesized from Phenylalanine. Its levels are typically low or normal, not elevated. * **Tryptophan:** Tryptophan levels are not elevated; in fact, its entry into the brain is blocked by high Phenylalanine, contributing to pathology. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Triad:** Intellectual disability, "Mousy" or "Musty" body odor (due to Phenylacetate), and hypopigmentation (fair skin/blue eyes) due to decreased melanin. * **Diagnosis:** Guthrie Test (bacterial inhibition assay) or Tandem Mass Spectrometry. * **Management:** Dietary restriction of Phenylalanine and supplementation of Tyrosine. Aspartame (artificial sweetener) must be avoided as it contains Phenylalanine. * **Maternal PKU:** If a mother with PKU doesn't maintain a strict diet during pregnancy, the fetus may develop microcephaly and congenital heart defects.

Practice by Chapter

Single Gene Disorders

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Biochemical Diagnosis of Genetic Disorders

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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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