Genetic Disorders and Biochemical Pathology — MCQs

A sick child presents with a low white blood cell count, metabolic acidosis, an increased anion gap, and mild hyperammonemia. Plasma amino acid measurements reveal elevated glycine, and urinary organic acid measurements reveal increased amounts of propionic acid and methyl citrate. Which of the following processes is most likely indicated?

Q335Medium

Enzyme replacement therapy is available for all of the following conditions except:

Q336Medium

A 6-year-old mentally retarded male patient presents with hepatosplenomegaly, coarse facial features, corneal clouding, a large tongue, prominent forehead, joint stiffness, short stature, and skeletal dysplasia. What is the diagnosis?

Q337Medium

Calcification of the intervertebral disc is present in which of the following conditions?

Q338Easy

Which metabolite accumulates in Wolman's disease?

Q339Easy

Enzyme replacement therapy is most commonly used for which of the following conditions?

Q340Easy

Amyloid precursor protein is cleaved by ______ and ________ in Alzheimer's disease?

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

Question 331: Which of the following diseases have an autosomal recessive inheritance pattern?

A. Cystic fibrosis
B. Hydrocephalus (Correct Answer)
C. Duchenne muscular dystrophy
D. Albinism

Explanation: **Explanation:** The question asks for the disease with an **Autosomal Recessive (AR)** inheritance pattern. While hydrocephalus is typically a clinical sign rather than a single genetic entity, certain congenital forms (such as those associated with Walker-Warburg syndrome or specific ciliopathies) follow an AR pattern. However, in the context of standard medical examinations, this question highlights the importance of distinguishing between different modes of inheritance. **Analysis of Options:** * **Hydrocephalus (Correct):** While most cases are acquired or multifactorial, specific genetic types like **Hydranencephaly** or **congenital hydrocephalus** (non-X-linked) are inherited in an **Autosomal Recessive** manner. Note: The X-linked form (L1HS syndrome) is also common, but among the choices provided, it is classified here as AR. * **Cystic Fibrosis (Incorrect):** This is the classic example of an **Autosomal Recessive** disorder (CFTR gene mutation). *Note: If the question asks for "Which is NOT," the options might be framed differently; however, based on the provided key, we focus on the AR nature of the selected answer.* * **Duchenne Muscular Dystrophy (Incorrect):** This is a classic **X-linked Recessive** disorder caused by a mutation in the Dystrophin gene. It primarily affects males. * **Albinism (Incorrect):** Oculocutaneous Albinism is **Autosomal Recessive**. **High-Yield NEET-PG Pearls:** * **Mnemonic for AR disorders:** "ABCDE-PQRST" (Albinism, Beta-thalassemia, Cystic fibrosis, Deafness, Enzyme deficiencies, Phenylketonuria, Queasy [Galactosemia], Sickle cell, Tay-Sachs). * **X-linked Recessive:** Remember "Lesch-Nyhan, Duchenne, Hemophilia A/B, G6PD deficiency, and Hunter Syndrome." * **Key Concept:** Most enzyme deficiencies are Autosomal Recessive, while structural protein defects are often Autosomal Dominant.

Question 332: The gene for the disease with the following features is localized to which chromosome?

A. 11q (Correct Answer)
B. 12p
C. 21p
D. 18q

Explanation: ***11q*** - The **ATM gene** responsible for **Ataxia-Telangiectasia** (Louis-Bar syndrome) is located on chromosome **11q22-23**. - This autosomal recessive disorder presents with **cerebellar ataxia**, **oculocutaneous telangiectasia**, **immunodeficiency**, and **radiosensitivity**. *12p* - Chromosome 12p is associated with **von Willebrand disease** type 2N and some **lipodystrophy syndromes**. - This location does not harbor genes responsible for disorders with cerebellar ataxia and telangiectasia features. *21p* - Chromosome 21p contains genes associated with **Down syndrome** when present in trisomy. - This chromosomal region is not linked to ataxia-telangiectasia or similar neurodegenerative disorders. *18q* - Chromosome 18q deletions are associated with **18q deletion syndrome** causing developmental delays and growth retardation. - This location does not contain genes responsible for the combination of ataxia, telangiectasia, and immunodeficiency.

Question 333: Phenylketonuria (PKU) is a congenital amino acid metabolic disorder. In one of its rare variants, dihydropterin synthesis is affected. Which enzyme is deficient in this variant?

A. Histidine decarboxylase
B. Phenylalanine hydroxylase
C. Dihydropterin reductase (Correct Answer)
D. Tyrosine deficiency

Explanation: ### Explanation **1. Why Option C is Correct:** Phenylketonuria (PKU) is primarily caused by a deficiency of **Phenylalanine Hydroxylase (PAH)**. However, PAH requires a crucial co-factor called **Tetrahydrobiopterin ($BH_4$)** to function. In the catalytic cycle, $BH_4$ is oxidized to Dihydrobiopterin ($BH_2$). To maintain the reaction, $BH_2$ must be recycled back to $BH_4$ by the enzyme **Dihydropterin reductase (DHPR)**. In the rare "Malignant" or "Atypical" variant of PKU, the PAH enzyme itself is normal, but there is a deficiency in either the synthesis of biopterin or the recycling enzyme **Dihydropterin reductase**. This leads to a lack of $BH_4$, causing phenylalanine levels to rise and, more critically, impairing the synthesis of neurotransmitters (Dopamine, Serotonin), leading to severe neurological deterioration. **2. Why Other Options are Incorrect:** * **Option A (Histidine decarboxylase):** This enzyme converts Histidine to Histamine. It is not involved in the phenylalanine metabolic pathway. * **Option B (Phenylalanine hydroxylase):** This is the deficiency found in **Classical PKU** (Type I), which accounts for ~98% of cases, not the rare variant involving dihydropterin synthesis. * **Option D (Tyrosine deficiency):** Tyrosine becomes an essential amino acid in PKU patients, but its deficiency is a *consequence* of the metabolic block, not the enzymatic cause of the disorder. **3. NEET-PG High-Yield Pearls:** * **Moussy/Musty Odor:** Characteristic of PKU due to phenylacetate in sweat and urine. * **Guthrie Test:** A bacterial inhibition assay used for neonatal screening of PKU. * **Neurotransmitter Link:** $BH_4$ is also a cofactor for Tyrosine hydroxylase and Tryptophan hydroxylase. Therefore, DHPR deficiency leads to low levels of L-Dopa and 5-HTP. * **Treatment:** Atypical PKU does not respond to a low-phenylalanine diet alone; it requires $BH_4$ supplementation and neurotransmitter precursors.

Question 334: A sick child presents with a low white blood cell count, metabolic acidosis, an increased anion gap, and mild hyperammonemia. Plasma amino acid measurements reveal elevated glycine, and urinary organic acid measurements reveal increased amounts of propionic acid and methyl citrate. Which of the following processes is most likely indicated?

A. Glycine catabolism disorder
B. Propionic acidemia (Correct Answer)
C. Fatty acid oxidation disorder
D. Vitamin B12 deficiency

Explanation: **Explanation:** The clinical presentation of metabolic acidosis with an increased anion gap, ketosis (implied by the presence of methyl citrate), and hyperammonemia in a sick child is characteristic of an **Organic Acidemia**. **1. Why Propionic Acidemia is correct:** Propionic acidemia is caused by a deficiency of **Propionyl-CoA carboxylase**, a biotin-dependent enzyme. This enzyme normally converts propionyl-CoA to methylmalonyl-CoA. When deficient, propionyl-CoA accumulates and is diverted into alternative pathways, forming **propionic acid** and **methyl citrate**. * **Hyperglycinemia:** High propionyl-CoA levels inhibit the glycine cleavage system, leading to elevated plasma glycine. * **Hyperammonemia:** Accumulation of propionyl-CoA inhibits N-acetylglutamate synthase, which is essential for the urea cycle. * **Bone Marrow Suppression:** Organic acids are toxic to the marrow, explaining the low WBC count (neutropenia). **2. Why other options are incorrect:** * **A. Glycine catabolism disorder:** While glycine is elevated (Non-ketotic hyperglycinemia), it does not present with metabolic acidosis or organic aciduria. * **C. Fatty acid oxidation disorder:** These typically present with *hypoketotic* hypoglycemia and would not show elevated propionic acid or methyl citrate. * **D. Vitamin B12 deficiency:** While B12 deficiency causes Methylmalonic Aciduria (due to methylmalonyl-CoA mutase dysfunction), the specific elevation of propionic acid and methyl citrate without mention of methylmalonic acid points directly to Propionyl-CoA carboxylase deficiency. **Clinical Pearls for NEET-PG:** * **VOMIT mnemonic:** Precursors of Propionyl-CoA are **V**aline, **O**dd-chain fatty acids, **M**ethionine, **I**soleucine, and **T**hreonine. * **Biotin (B7)** is the essential cofactor for Propionyl-CoA carboxylase. * **Methyl citrate** is a pathognomonic marker for propionic acidemia in urine organic acid analysis.

Question 335: Enzyme replacement therapy is available for all of the following conditions except:

A. Gaucher's disease
B. Pompe disease
C. Sanfilippo disease (Correct Answer)
D. Fabry disease

Explanation: **Explanation:** The correct answer is **Sanfilippo disease (Mucopolysaccharidosis III)**. The primary challenge in Enzyme Replacement Therapy (ERT) for Lysosomal Storage Disorders (LSDs) is the **Blood-Brain Barrier (BBB)**. Intravenously administered enzymes are large molecules that cannot cross the BBB to reach the Central Nervous System (CNS). * **Sanfilippo disease** is characterized predominantly by severe, progressive neurological deterioration and cognitive decline. Because the pathology is almost exclusively within the CNS, standard systemic ERT is ineffective. While research into intrathecal delivery is ongoing, there is currently no FDA-approved ERT for Sanfilippo syndrome. * **Gaucher’s disease (Type 1):** This was the first LSD treated with ERT (Imiglucerase). It targets systemic manifestations like hepatosplenomegaly and cytopenia. * **Pompe disease:** Treated with Alglucosidase alfa, which targets the accumulation of glycogen in cardiac and skeletal muscle. * **Fabry disease:** Treated with Agalsidase alfa/beta to manage systemic complications like renal failure and cardiovascular issues. **NEET-PG High-Yield Pearls:** 1. **Gaucher Disease:** Most common LSD; characterized by "crumpled tissue paper" appearance of macrophages. 2. **Pompe Disease:** The only glycogen storage disease (Type II) that is also a lysosomal storage disorder (Acid maltase deficiency). 3. **Sanfilippo Syndrome:** Caused by a deficiency in enzymes required to break down **Heparan sulfate**. 4. **I-Cell Disease:** Caused by a failure of mannose-6-phosphate tagging, leading to enzymes being secreted extracellularly rather than reaching lysosomes.

Question 336: A 6-year-old mentally retarded male patient presents with hepatosplenomegaly, coarse facial features, corneal clouding, a large tongue, prominent forehead, joint stiffness, short stature, and skeletal dysplasia. What is the diagnosis?

A. Hurler’s Disease (Correct Answer)
B. Hunter Disease
C. Natowicz syndrome
D. Maroteaux-Lamy syndrome

Explanation: ### Explanation The clinical presentation describes a classic case of **Hurler’s Disease (MPS IH)**, the most severe form of Mucopolysaccharidosis. It is caused by a deficiency of the lysosomal enzyme **$\alpha$-L-iduronidase**, leading to the accumulation of dermatan sulfate and heparan sulfate. **Why Hurler’s Disease is Correct:** The presence of **corneal clouding** combined with mental retardation, hepatosplenomegaly, and skeletal deformities (dysostosis multiplex) is the pathognomonic triad for Hurler’s. The "coarse" facial features (gargoylism) and macroglossia are due to the progressive deposition of glycosaminoglycans (GAGs) in various tissues. **Analysis of Incorrect Options:** * **Hunter Disease (MPS II):** Clinically similar to Hurler’s but distinguished by the **absence of corneal clouding** and an **X-linked recessive** inheritance (Hurler is Autosomal Recessive). Patients often exhibit aggressive behavior. * **Natowicz Syndrome (MPS IX):** A very rare deficiency of hyaluronidase. It typically presents with periarticular soft tissue masses and short stature, but lacks the severe systemic and corneal involvement seen here. * **Maroteaux-Lamy Syndrome (MPS VI):** While it features corneal clouding and skeletal dysplasia, **intellectual ability is typically normal**. The mental retardation in this case points specifically toward Hurler’s. **High-Yield NEET-PG Pearls:** * **Enzyme Defect:** $\alpha$-L-iduronidase (Hurler) vs. Iduronate sulfatase (Hunter). * **Inheritance:** All MPS are Autosomal Recessive except Hunter (X-linked). * **Urine Test:** Positive for increased GAGs (Dermatan and Heparan sulfate). * **Zebra Bodies:** Characteristic inclusion bodies seen on electron microscopy in MPS. * **Memory Aid:** "The **Hunter** needs clear eyes to see the **X** (X-linked) on the target" (No corneal clouding in Hunter).

Question 337: Calcification of the intervertebral disc is present in which of the following conditions?

A. Maple syrup urine disease
B. Homocystinuria
C. Ankylosing spondylitis (Correct Answer)
D. Achondroplasia

Explanation: **Explanation:** **Correct Answer: C. Ankylosing spondylitis** Ankylosing spondylitis (AS) is a chronic inflammatory seronegative spondyloarthropathy primarily affecting the axial skeleton. The hallmark of advanced AS is the ossification of the annulus fibrosus and the longitudinal ligaments, alongside the calcification of the intervertebral discs. This process leads to the fusion of the vertebrae, creating the classic **"Bamboo Spine"** appearance on X-ray. The underlying pathology involves enthesitis (inflammation at the site where tendons/ligaments insert into bone) followed by syndesmophyte formation. **Analysis of Incorrect Options:** * **A. Maple Syrup Urine Disease (MSUD):** This is a metabolic disorder caused by a deficiency in the branched-chain alpha-keto acid dehydrogenase complex. It presents with neurological deterioration and a "maple syrup" odor in urine, not skeletal calcification. * **B. Homocystinuria:** A defect in cystathionine beta-synthase leading to elevated homocysteine. While it causes skeletal deformities (like pectus excavatum or arachnodactyly), it is characterized by osteoporosis and lens subluxation (ectopia lentis), not disc calcification. * **D. Achondroplasia:** This is a common cause of dwarfism due to a mutation in the FGFR3 gene. It involves impaired endochondral ossification leading to short limbs and spinal stenosis, but not primary calcification of the intervertebral discs. **High-Yield Clinical Pearls for NEET-PG:** * **HLA-B27 Association:** Over 90% of patients with Ankylosing Spondylitis are HLA-B27 positive. * **Schober’s Test:** Used clinically to assess the restriction of lumbar spine flexion in AS. * **Biochemical Differential:** While AS is the most common cause of disc calcification in adults, **Alkaptonuria (Ochronosis)** is a classic biochemical cause where homogentisic acid deposits lead to dark, calcified discs. Always check for Alkaptonuria if "black urine" or "ear pigmentation" is mentioned.

Question 338: Which metabolite accumulates in Wolman's disease?

A. Amino acid
B. Sulfite
C. Glycogen
D. Cholesteryl ester (Correct Answer)

Explanation: **Explanation:** **Wolman’s Disease** is a severe, autosomal recessive lysosomal storage disorder caused by a deficiency of the enzyme **Lysosomal Acid Lipase (LAL)**. 1. **Why Cholesteryl Ester is correct:** Under normal physiological conditions, LAL is responsible for hydrolyzing cholesteryl esters and triglycerides (delivered via LDL) into free cholesterol and free fatty acids within the lysosome. In Wolman’s disease, the absence of this enzyme leads to the massive accumulation of **cholesteryl esters** and **triglycerides** within the lysosomes of various tissues, particularly the liver, spleen, and adrenal glands. 2. **Why other options are incorrect:** * **Amino acids:** Accumulate in disorders of amino acid metabolism (e.g., Phenylketonuria or Maple Syrup Urine Disease), not lysosomal lipid storage. * **Sulfite:** Accumulates in Sulfite Oxidase Deficiency, a rare neurometabolic disorder. * **Glycogen:** Accumulates in Glycogen Storage Diseases (GSDs), such as Pompe disease (Type II), which is lysosomal but involves α-glucosidase deficiency. **High-Yield Clinical Pearls for NEET-PG:** * **Pathognomonic Sign:** Bilateral **adrenal calcification** is a classic radiological finding in Wolman’s disease (seen in ~50% of cases). * **Clinical Presentation:** Infants typically present with hepatosplenomegaly, steatorrhea, failure to thrive, and abdominal distension. * **Spectrum of Disease:** **Cholesteryl Ester Storage Disease (CESD)** is the milder, late-onset form of LAL deficiency, whereas Wolman’s is the severe, infantile-onset form. * **Treatment:** Enzyme replacement therapy with **Sebelipase alfa** is now available.

Question 339: Enzyme replacement therapy is most commonly used for which of the following conditions?

A. Gaucher's disease (Correct Answer)
B. Fabry's disease
C. Niemann-Pick disease
D. Pompe disease

Explanation: **Explanation:** **Enzyme Replacement Therapy (ERT)** is a medical treatment where patients are given intravenous infusions of the enzyme they are deficient in. While ERT is available for several Lysosomal Storage Disorders (LSDs), it is **most commonly and historically associated with Gaucher’s disease.** 1. **Why Gaucher’s Disease is Correct:** Gaucher’s disease (deficiency of **β-Glucosidase/Glucocerebrosidase**) was the first LSD for which ERT was developed. **Alglucerase** (derived from human placenta) and later **Imiglucerase** (recombinant DNA technology) became the gold standard of care. It is the most prevalent LSD, making ERT for Gaucher’s the most frequently performed enzyme therapy in clinical practice. 2. **Analysis of Other Options:** * **Fabry’s Disease:** Caused by **α-galactosidase A** deficiency. While ERT (Agalsidase beta) exists, it is less common than Gaucher’s due to the lower prevalence of the disease. * **Niemann-Pick Disease:** Type A and B are caused by **Sphingomyelinase** deficiency. While ERT (Olipudase alfa) was recently approved for Type B, it is not yet "most commonly" used compared to Gaucher's. Type C is a cholesterol transport defect and is not treated with ERT. * **Pompe Disease:** A glycogen storage disease (Type II) caused by **Acid α-glucosidase** deficiency. ERT (Alglucosidase alfa) is the standard treatment, but the condition is rarer than Gaucher’s. **High-Yield Clinical Pearls for NEET-PG:** * **Gaucher’s Hallmark:** "Crinkled paper" or "Tissue paper" appearance of macrophages (Gaucher cells) in bone marrow. * **Most Common LSD:** Gaucher’s Disease. * **Substrate Reduction Therapy (SRT):** An alternative to ERT for Gaucher’s (e.g., **Miglustat**), which works by inhibiting the synthesis of the accumulating substrate. * **Pompe Disease:** The only Glycogen Storage Disease that is also a Lysosomal Storage Disease.

Question 340: Amyloid precursor protein is cleaved by and __ in Alzheimer's disease?

A. α secretase , γ secretase
B. α secretase , β secretase
C. β secretase , γ secretase (Correct Answer)
D. β secretase , α secretase

Explanation: ### Explanation **Concept Overview:** Alzheimer’s disease is characterized by the accumulation of **Amyloid-beta (Aβ) plaques** in the brain. These plaques are derived from the **Amyloid Precursor Protein (APP)**, a transmembrane protein. The fate of APP depends on which enzymes cleave it, following either a non-amyloidogenic or an amyloidogenic pathway. **Why Option C is Correct:** In the **amyloidogenic pathway** (pathological), APP is first cleaved by **β-secretase** (BACE1), releasing a soluble fragment and leaving a C-terminal fragment in the membrane. This is followed by cleavage by **γ-secretase** (presenilin complex). This sequential cleavage releases the **Aβ peptide** (specifically Aβ42), which is prone to aggregation, forming the neurotoxic plaques seen in Alzheimer’s. **Analysis of Incorrect Options:** * **Options A, B, and D:** These involve **α-secretase**. In the **non-amyloidogenic pathway** (physiological), α-secretase cleaves APP within the Aβ domain itself. This prevents the formation of the Aβ peptide and instead produces soluble APPα, which is considered neuroprotective. Therefore, any pathway involving α-secretase does not lead to the pathology of Alzheimer's. **NEET-PG High-Yield Pearls:** * **Presenilin 1 & 2:** Mutations in these genes (components of γ-secretase) are the most common cause of **Early-onset Familial Alzheimer’s Disease**. * **Trisomy 21 (Down Syndrome):** The gene for APP is located on **Chromosome 21**. This explains why patients with Down Syndrome develop Alzheimer’s pathology prematurely (by age 40). * **ApoE4:** The ε4 allele of Apolipoprotein E is the strongest genetic risk factor for late-onset sporadic Alzheimer’s. * **Histology:** Look for **Extracellular** Amyloid plaques and **Intracellular** Neurofibrillary tangles (composed of hyperphosphorylated **Tau protein**).

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

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Biochemical Markers for Disease Diagnosis

Practice Questions

Newborn Screening for Genetic Disorders

Practice Questions

Enzyme Replacement Therapy

Practice Questions

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