Genetic Disorders and Biochemical Pathology Practice Questions

Q: Which of the following is NOT a single gene disorder?

William's syndrome. **Explanation:** The distinction between single-gene (Mendelian) disorders and chromosomal microdeletion syndromes is a high-yield topic in NEET-PG Biochemistry and Genetics. **Why William’s Syndrome is the correct answer:** Unlike the other options, **William’s Syndrome** is not caused by a mutation in a single gene. It is a **microdeletion syndrome** involving the contiguous loss of approximately 26–28 genes on the long arm of **chromosome 7 (7q11.23)**. The most notable gene lost in this region is the *ELN* (elastin) gene, which accounts for the cardiovascular features, but the overall phenotype results from the loss of multiple neighboring genes. **Analysis of Incorrect Options (Single Gene Disorders):** * **Haemochromatosis:** An **Autosomal Recessive** disorder typically caused by a single mutation in the *HFE* gene (most commonly C282Y), leading to iron overload. * **Cystic Fibrosis:** An **Autosomal Recessive** disorder caused by mutations in the *CFTR* gene on chromosome 7. The most common mutation is the deletion of phenylalanine at position 508 (ΔF508). * **Huntington’s Disease:** An **Autosomal Dominant** trinucleotide repeat disorder (CAG repeats) in the *HTT* gene on chromosome 4. **Clinical Pearls for NEET-PG:** * **William’s Syndrome Triad:** "Elfin" facies, Intellectual disability with "cocktail party personality" (extreme friendliness), and **Supravalvular Aortic Stenosis (SVAS)**. * **Diagnosis:** The gold standard for microdeletion syndromes like William's is **FISH** (Fluorescence In Situ Hybridization) or Microarray, not standard karyotyping. * **Hypercalcemia:** Patients often present with idiopathic infantile hypercalcemia due to increased sensitivity to Vitamin D.

Q: Which of the following is not a channelopathy?

Tay-sachs disease. **Explanation:** **Channelopathies** are a group of heterogeneous disorders caused by mutations in genes encoding ion channel proteins or their interacting proteins. These defects disrupt the flow of ions (like Na+, K+, Cl-, or Ca2+) across cell membranes. **Why Tay-Sachs Disease is the Correct Answer:** Tay-Sachs disease is **not** a channelopathy; it is a **Lysosomal Storage Disorder**. It is caused by a deficiency of the enzyme **Hexosaminidase A**, leading to the toxic accumulation of **GM2 gangliosides** in neurons. It typically presents with neurodegeneration, developmental delay, and a characteristic "cherry-red spot" on the macula. **Analysis of Incorrect Options (Channelopathies):** * **Cystic Fibrosis:** Caused by mutations in the **CFTR gene**, which encodes a cAMP-regulated **chloride channel**. It is one of the most common fatal genetic channelopathies. * **Liddle’s Syndrome:** An autosomal dominant condition involving a "gain-of-function" mutation in the **ENaC (Epithelial Sodium Channel)** in the renal collecting ducts, leading to hypertension and hypokalemia. * **Hypokalemic Periodic Paralysis:** Most commonly caused by mutations in the **CACNA1S** (Calcium channel) or **SCN4A** (Sodium channel) genes in skeletal muscle [1]. **NEET-PG High-Yield Pearls:** * **Cardiac Channelopathies:** Long QT Syndrome (K+ channel), Brugada Syndrome (Na+ channel). * **Neurological Channelopathies:** Familial Hemiplegic Migraine, Episodic Ataxia. * **Endocrine Channelopathy:** Neonatal Diabetes/PHHI (K-ATP channels). * **Tay-Sachs Mnemonic:** "A **Gang** of **Six** (**Hex**) **S**mall (**S**achs) **Lycans** (**Lysosomal**)." (GM2 Ganglioside, Hexosaminidase A, Lysosomal).

Q: A 12-year-old girl presents with tremors and emotional lability. Slit lamp examination of the eye shows golden brown discoloration in Descemet's membrane. What is the most appropriate diagnosis for this patient?

Wilson's disease. ### Explanation **Correct Answer: C. Wilson’s Disease** **1. Why it is correct:** The clinical triad of **neuropsychiatric symptoms** (tremors, emotional lability), **hepatic involvement**, and the pathognomonic **Kayser-Fleischer (KF) rings** is diagnostic of Wilson’s disease. KF rings are golden-brown deposits of copper in the **Descemet’s membrane** of the cornea, best visualized via slit-lamp examination. Biochemically, Wilson’s disease is an autosomal recessive disorder caused by mutations in the **ATP7B gene** (Chromosome 13). This leads to defective biliary copper excretion and failure to incorporate copper into ceruloplasmin, resulting in toxic copper accumulation in the liver, brain (basal ganglia), and eyes. **2. Why other options are incorrect:** * **A. Acute Rheumatic Fever:** While it can present with Sydenham’s chorea (involuntary movements), it is associated with streptococcal infections and carditis, not corneal copper deposits. * **B. Fabry’s Disease:** An X-linked lysosomal storage disorder. While it involves the eye, it presents with **cornea verticillata** (vortex keratopathy), not KF rings, and typically involves angiokeratomas and renal failure. * **D. Glycogen Storage Disease:** These primarily present with hypoglycemia, hepatomegaly, or muscular weakness, depending on the type (e.g., Von Gierke’s), but do not cause Descemet’s membrane discoloration. **3. NEET-PG High-Yield Pearls:** * **Best Initial Test:** Serum Ceruloplasmin (decreased; 250 µg/g dry weight). * **Most Sensitive Screening:** 24-hour urinary copper excretion (increased). * **Treatment of Choice:** Chelating agents like **D-Penicillamine** or Trientine; Zinc is used for maintenance (inhibits GI absorption). * **MRI Finding:** "Face of the Giant Panda" sign in the midbrain.

Q: All of the following types of renal stones are seen in females, EXCEPT:

Renal stones in Lesch-Nyhan syndrome. **Explanation:** The correct answer is **D. Renal stones in Lesch-Nyhan syndrome**. **Why it is the correct answer:** Lesch-Nyhan syndrome is an **X-linked recessive** disorder caused by a deficiency of the enzyme hypoxanthine-guanine phosphoribosyltransferase (HGPRT). Because it is X-linked recessive, the clinical phenotype—characterized by hyperuricemia, gout, self-mutilation, and **uric acid renal stones**—is seen almost exclusively in **males**. Females are typically asymptomatic carriers. **Why the other options are incorrect:** * **A. Idiopathic hypercalciuria:** This is the most common metabolic abnormality in stone formers. It follows an autosomal dominant inheritance pattern or is multifactorial, affecting both males and females. * **B. Distal Renal Tubular Acidosis (Type 1 RTA):** This condition leads to alkaline urine, hypocitraturia, and hypercalciuria, resulting in nephrocalcinosis and calcium phosphate stones. It affects both sexes equally. * **C. Primary hyperoxaluria:** This is an autosomal recessive genetic disorder leading to excessive oxalate production and calcium oxalate stones. As an autosomal condition, it occurs in both males and females. **NEET-PG High-Yield Pearls:** * **Lesch-Nyhan Syndrome:** Look for the triad of "Gout, Intellectual disability, and Self-mutilation" in a male child. * **Staghorn Calculi:** More common in females due to the higher incidence of Proteus-induced Urinary Tract Infections (UTIs) and struvite stones. * **Most common stone overall:** Calcium oxalate (specifically Calcium oxalate monohydrate/Whewellite). * **Stone Radiopacity:** Most stones are radiopaque; **Pure Uric Acid** and **Indinavir** stones are classically radiolucent on X-ray.

Q: Congenital bilateral absence of vas deferens is genetically related to which of the following conditions?

Cystic fibrosis. **Explanation:** **Cystic Fibrosis (CF)** is the correct answer because **Congenital Bilateral Absence of the Vas Deferens (CBAVD)** is a highly specific clinical feature of this disorder. CF is an autosomal recessive condition caused by mutations in the **CFTR gene** (Chromosome 7), which encodes a chloride channel. In males, defective CFTR leads to thickened secretions that obstruct the developing vas deferens *in utero*, causing them to involute or fail to develop. Notably, up to 95% of males with CF are infertile due to CBAVD, and some individuals may present with CBAVD as their *only* clinical manifestation of a CFTR mutation. **Analysis of Incorrect Options:** * **Noonan Syndrome:** Characterized by short stature, webbed neck, and pulmonary stenosis. While cryptorchidism (undescended testes) is common, CBAVD is not a feature. * **Down Syndrome (Trisomy 21):** Associated with intellectual disability, cardiac defects (AVSD), and early-onset Alzheimer’s. While fertility may be reduced, the anatomy of the vas deferens is typically intact. * **Kartagener Syndrome:** A subset of Primary Ciliary Dyskinesia (PCD). It causes male infertility due to **immotile spermatozoa** (dynein arm defect), but the vas deferens is anatomically present. **NEET-PG High-Yield Pearls:** * **Genetics:** Most common mutation in CF is **ΔF508** (phenylalanine deletion), leading to protein misfolding and degradation in the ER. * **Diagnosis:** Sweat chloride test (>60 mEq/L) is the gold standard. * **CBAVD vs. Kartagener:** In exams, if the patient has obstructive azoospermia (absent vas), think **CF**; if they have normal sperm count but zero motility, think **Kartagener**. * **Associated finding:** CBAVD is often associated with seminal vesicle hypoplasia and low semen volume (<1 mL) with acidic pH.

Q: Cardiac manifestations can be seen in all of the following metabolic diseases EXCEPT?

Galactosemia. **Explanation:** The correct answer is **Galactosemia**. This metabolic disorder primarily affects the liver, eyes, and brain, but it is **not** typically associated with cardiac pathology. **1. Why Galactosemia is the correct answer:** Classic Galactosemia (deficiency of Galactose-1-phosphate uridyltransferase or GALT) leads to the accumulation of galactose-1-phosphate and galactitol. The clinical triad includes **Hepatomegaly/Jaundice**, **Early-onset Cataracts** (due to galactitol accumulation in the lens), and **Intellectual Disability**. A critical high-yield feature is the predisposition to **E. coli sepsis**. Cardiac involvement (like cardiomyopathy or arrhythmias) is absent in this condition. **2. Why the other options are incorrect:** * **Fatty Acid Oxidation Defects (e.g., MCAD/LCAD deficiency):** The heart relies heavily on fatty acid oxidation for energy. Defects in this pathway lead to energy failure in myocytes, resulting in **Hypertrophic or Dilated Cardiomyopathy** and arrhythmias. * **Mitochondrial Disorders (e.g., MELAS, Kearns-Sayre):** Tissues with high metabolic demands, like the heart and brain, are most affected. These often present with **Conduction blocks** and cardiomyopathy. * **Fabry Disease:** This is a lysosomal storage disorder (alpha-galactosidase A deficiency) where glycosphingolipids deposit in the vascular endothelium and myocardium, leading to **Left Ventricular Hypertrophy (LVH)**, heart failure, and MI at a young age. **Clinical Pearls for NEET-PG:** * **Pompe Disease (GSD II):** The classic metabolic cause of massive cardiomegaly ("Profoundly enlarged heart") in an infant. * **Galactosemia Screening:** Look for "Reducing sugars in urine" (Clinitest positive) but "Glucose oxidase test" (Dipstick) negative. * **Dietary Management:** Immediate withdrawal of lactose/galactose (Soy-based formula).

Q: Amelogenesis imperfecta is due to a defect in which of the following?

Enamel matrix proteins (e.g., amelogenin). **Explanation:** **Amelogenesis Imperfecta (AI)** is a group of hereditary disorders characterized by abnormal enamel formation in the absence of systemic features. The correct answer is **Enamel matrix proteins (Option A)** because enamel is derived from epithelial cells (ameloblasts) that secrete a specific protein matrix which then mineralizes. * **Amelogenin (AMELX):** This is the most abundant enamel matrix protein (approx. 90%). Mutations in the *AMELX* gene on the X-chromosome are a leading cause of X-linked AI. * **Other proteins:** Mutations in **Enamelysin (MMP20)**, **Enamelin (ENAM)**, and **Kallikrein-4 (KLK4)** also lead to various subtypes (hypoplastic, hypocalcified, or hypomaturation) of AI. **Analysis of Incorrect Options:** * **DSPP (Option C):** Mutations in the *Dentin Sialophosphoprotein* gene lead to **Dentinogenesis Imperfecta (Type II and III)** and Dentin Dysplasia. DSPP is specific to the mesenchymal-derived dentin matrix, not enamel. * **MSX-1 and PAX-9 (Options B & D):** These are transcription factors involved in early tooth signaling and morphogenesis. Mutations in these genes are typically associated with **Hypodontia** (congenitally missing teeth) or **Oligodontia**, rather than structural defects of the enamel itself. **High-Yield Clinical Pearls for NEET-PG:** * **Enamel vs. Dentin:** Enamel is **ectodermal** in origin; Dentin is **mesodermal** (ectomesenchymal). * **Radiographic Appearance:** In AI, the enamel may be thin or have the same radiodensity as dentin, leading to a loss of the normal "contrast" between the two layers. * **Associated Syndrome:** If AI is seen with gingival fibromatosis, consider **Rutherford Syndrome**. If seen with nephrocalcinosis, consider **Enamel-Renal Syndrome** (FAM20A mutation).

Q: All are chromosomal trisomies except?

Turner syndrome. ### Explanation The core concept of this question lies in distinguishing between **Autosomal Trisomies** (gain of a chromosome) and **Sex Chromosome Monosomies** (loss of a chromosome). **Why Turner Syndrome is the correct answer:** Turner syndrome is a condition characterized by **Monosomy X (45, XO)**. Unlike the other options, it involves the *absence* of a second sex chromosome rather than the addition of an extra one. It is the most common sex chromosome abnormality in females and is not a trisomy. **Analysis of Incorrect Options (Trisomies):** * **Down Syndrome (Option A):** This is **Trisomy 21**. It is the most common autosomal trisomy among live births. * **Patau Syndrome (Option B):** This is **Trisomy 13**. It is characterized by severe midline defects like cleft lip/palate, holoprosencephaly, and polydactyly. * **Edward Syndrome (Option D):** This is **Trisomy 18**. Clinical hallmarks include "rocker-bottom feet," clenched fists with overlapping fingers, and micrognathia. **High-Yield NEET-PG Clinical Pearls:** 1. **Mnemonic for Trisomies:** * **P**atau = **P**uberty (Age **13**) * **E**dward = **E**lection (Age **18**) * **D**own = **D**rinking age (Age **21**) 2. **Turner Syndrome Features:** Short stature, webbed neck (cystic hygroma), streak ovaries (primary amenorrhea), and coarctation of the aorta. 3. **Most common cause of Trisomies:** Meiotic non-disjunction, strongly associated with advanced maternal age. 4. **Most common cause of Turner Syndrome:** Paternal meiotic non-disjunction (loss of the paternal X chromosome).

Q: Blood spot tandem mass spectrometry can be used in the diagnosis of all the following disorders EXCEPT?

Lesch Nyhan disease. **Explanation:** **Tandem Mass Spectrometry (TMS)** is the gold standard for newborn screening because it can simultaneously detect a wide range of metabolites, specifically **acylcarnitines** and **amino acids**, from a single dried blood spot. 1. **Why Lesch-Nyhan Disease is the correct answer:** Lesch-Nyhan syndrome is an X-linked recessive disorder caused by a deficiency of the enzyme **HGPRT**, leading to the overproduction of uric acid. Diagnosis is primarily based on clinical presentation (self-mutilation, gout, choreoathetosis), hyperuricemia, and confirmed via **enzyme assay** or **molecular genetic testing**. It does not produce the specific acylcarnitine or amino acid profiles detectable by standard TMS screening protocols. 2. **Why the other options are incorrect:** * **Fatty acid oxidation defects (e.g., MCAD deficiency):** These are the hallmark disorders diagnosed by TMS. It detects specific **acylcarnitine** patterns that accumulate when fats cannot be broken down. * **Mitochondrial disorders:** While broad, many primary mitochondrial diseases (like respiratory chain defects) result in secondary elevations of specific metabolites (like C4-DC or C5-OH acylcarnitines) detectable via TMS. * **Propionic Acidemia:** This is an **Organic Acidemia**. TMS detects the elevation of **Propionylcarnitine (C3)**, making it a routine part of the TMS screening panel. **NEET-PG High-Yield Pearls:** * **TMS Utility:** "One test, many diseases." It is used for Aminoacidopathies (PKU, MSUD), Organic Acidemias (MMA, PA), and Fatty Acid Oxidation Disorders (MCAD, VLCAD). * **Lesch-Nyhan Key Feature:** Look for "Orange sand" in the diaper (urates) and self-mutilating behavior. * **Guthrie Test:** An older, bacterial inhibition assay used specifically for PKU; now largely replaced by TMS.

Question 1

Which of the following is an exception to mitochondrial inheritance?

Accepted Answer

Colour blindness

Answer

Leigh disease

Answer

Mitochondrial encephalopathy

Answer

MELAS

Question 2

Which of the following is NOT a single gene disorder?

Accepted Answer

William's syndrome

Answer

Haemochromatosis

Answer

Cystic fibrosis

Answer

Huntington's disease

Question 3

Which of the following is not a channelopathy?

Accepted Answer

Tay-sachs disease

Answer

Cystic fibrosis

Answer

Liddle's syndrome

Answer

Hypokalemic periodic paralysis

Question 4

A 12-year-old girl presents with tremors and emotional lability. Slit lamp examination of the eye shows golden brown discoloration in Descemet's membrane. What is the most appropriate diagnosis for this patient?

Accepted Answer

Wilson's disease

Answer

Acute Rheumatic fever

Answer

Fabry's disease

Answer

Glycogen storage disease

Question 5

All of the following types of renal stones are seen in females, EXCEPT:

Accepted Answer

Renal stones in Lesch-Nyhan syndrome

Answer

Stones associated with Idiopathic hypercalciuria

Answer

Renal stones in Distal renal tubular acidosis

Answer

Renal stones in Primary hyperoxaluria

Question 6

Congenital bilateral absence of vas deferens is genetically related to which of the following conditions?

Accepted Answer

Cystic fibrosis

Answer

Noonan syndrome

Answer

Down syndrome

Answer

Kartagener syndrome

Question 7

Cardiac manifestations can be seen in all of the following metabolic diseases EXCEPT?

Accepted Answer

Galactosemia

Answer

Fatty acid oxidation defects

Answer

Mitochondrial disorders

Answer

Fabry disease

Question 8

Amelogenesis imperfecta is due to a defect in which of the following?

Accepted Answer

Enamel matrix proteins (e.g., amelogenin)

Answer

MSX-1 (Msh Homeobox 1)

Answer

DSPP (Dentin Sialophosphoprotein)

Answer

PAX-9 (Paired Box 9)

Question 9

All are chromosomal trisomies except?

Accepted Answer

Turner syndrome

Answer

Down syndrome

Answer

Patau syndrome

Answer

Edward syndrome

Question 10

Blood spot tandem mass spectrometry can be used in the diagnosis of all the following disorders EXCEPT?

Accepted Answer

Lesch Nyhan disease

Answer

Fatty acid oxidation defects

Answer

Mitochondrial disorders

Answer

Propionic Acidemia

Genetic Disorders and Biochemical Pathology — MCQs

Genetic Disorders and Biochemical Pathology — MCQs

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