Genetic Disorders and Biochemical Pathology Practice Questions

Q: Which of the following is an X-linked dominant disorder?

Vitamin D resistant rickets. **Explanation:** **Vitamin D Resistant Rickets (Hypophosphatemic Rickets)** is the classic prototype of an **X-linked dominant (XLD)** disorder. It is caused by mutations in the *PHEX* gene on the X chromosome, leading to impaired phosphate reabsorption in the kidneys. In XLD inheritance, a single copy of the mutated gene on the X chromosome is sufficient to cause the disease in both males and females. A key pedigree feature is that an affected father will pass the trait to **all** of his daughters but **none** of his sons. **Analysis of Incorrect Options:** * **Fragile X Syndrome:** While often associated with X-linked inheritance, it is technically classified as an **X-linked dominant disorder with variable expressivity/incomplete penetrance**. However, in standard medical examinations, Vitamin D resistant rickets is the more "pure" and frequently tested example of XLD. * **Duchenne Muscular Dystrophy (DMD):** This is a classic **X-linked recessive** disorder. It primarily affects males, while females are typically asymptomatic carriers. * **Myotonic Dystrophy:** This is an **Autosomal Dominant** disorder characterized by trinucleotide repeat expansion (CTG) and the phenomenon of anticipation. **NEET-PG High-Yield Pearls:** * **X-linked Dominant Mnemonics:** Remember "**F**ragile **A**lport's **V**itamin **I**ncontinentia" (**F**ragile X, **A**lport Syndrome, **V**itamin D resistant rickets, **I**ncontinentia Pigmenti). * **Clinical Marker:** In Vitamin D resistant rickets, look for low serum phosphate, normal serum calcium, and high urinary phosphate despite normal Vitamin D intake. * **Pedigree Rule:** If a father is affected and has a healthy daughter, the disorder **cannot** be X-linked dominant.

Q: A clinical study of adults with a body mass index of at least 30 is undertaken. About 8% of these individuals do not have hyperphagia but are found to have normal levels of leptin and ghrelin, along with a diminished basal metabolic rate. A mutation in which of the following genes is most likely present in these individuals?

MC4R. **Explanation:** The correct answer is **MC4R (Melanocortin 4 Receptor)**. **1. Why MC4R is correct:** The **Melanocortin-4 Receptor (MC4R)** is a critical component of the leptin-melanocortin signaling pathway in the hypothalamus, which regulates energy homeostasis and food intake. Mutations in the *MC4R* gene are the **most common monogenic cause of human obesity**, accounting for up to 5-8% of cases of severe adult obesity. Unlike leptin or POMC deficiencies, which typically present with early-onset extreme hyperphagia, MC4R mutations can present with a **diminished basal metabolic rate (BMR)** and a more variable appetite profile in adulthood. Since the defect is at the receptor level (downstream), levels of upstream hormones like **Leptin** and **Ghrelin** remain normal. **2. Why other options are incorrect:** * **OB (Leptin) & OB-R (Leptin Receptor):** Mutations in the *OB* gene (causing leptin deficiency) or *OB-R* gene (causing leptin resistance) result in **extreme hyperphagia** and early-onset morbid obesity. In *OB* mutations, leptin levels would be undetectable, not normal. * **POMC (Pro-opiomelanocortin):** POMC is a precursor for $\alpha$-MSH, which activates MC4R. POMC deficiency causes severe obesity and hyperphagia, but it is classically associated with **hypoadrenalism** (due to lack of ACTH) and **red hair/pale skin** (due to lack of MSH). **Clinical Pearls for NEET-PG:** * **MC4R mutation:** Most common monogenic obesity; inherited in an autosomal dominant fashion. * **Leptin:** Produced by adipocytes (the "satiety hormone"); acts on the arcuate nucleus to inhibit NPY/AgRP (orexigenic) and stimulate POMC/CART (anorexigenic) neurons. * **Ghrelin:** Produced by P/D1 cells of the stomach fundus; the only peripheral orexigenic (appetite-stimulating) hormone. * **Prader-Willi Syndrome:** The most common *syndromic* cause of obesity, characterized by high ghrelin levels.

Q: Which of the following statements about cystinosis is incorrect?

Cystine stones are commonly found in the urine.. ### Explanation **Cystinosis** is a rare autosomal recessive lysosomal storage disorder caused by a mutation in the **CTNS gene**, which encodes **cystinosin**, a lysosomal membrane transporter. This leads to the accumulation of free cystine within lysosomes across various tissues. **1. Why Option A is the Correct (Incorrect Statement):** In cystinosis, cystine accumulates **intracellularly** within lysosomes. It does not spill over into the urine in high enough concentrations to form stones. **Cystine stones** are instead the hallmark of **Cystinuria**, a defect in the renal tubular amino acid transporter (COAL) leading to high urinary excretion of cystine. Therefore, the presence of stones is characteristic of Cystinuria, not Cystinosis. **2. Analysis of Other Options:** * **Option B (Corneal crystals):** Deposition of cystine crystals in the cornea is a pathognomonic feature, often leading to severe photophobia and visual impairment. * **Option C (Fanconi syndrome):** The accumulation of cystine in the proximal convoluted tubule (PCT) cells causes cellular damage, leading to Renal Fanconi Syndrome (generalized malabsorption of glucose, amino acids, uric acid, and phosphates). * **Option D (White/Blond hair and Photophobia):** Patients often exhibit hypopigmentation (fair skin and blond hair) due to altered melanin metabolism. Photophobia occurs due to the aforementioned corneal crystal deposits. **Clinical Pearls for NEET-PG:** * **Cystinosis:** Intracellular (lysosomal) accumulation; causes Fanconi syndrome and Renal Failure. * **Cystinuria:** Extracellular (urinary) accumulation; causes Hexagonal crystals and staghorn calculi. * **Diagnosis:** Confirmed by measuring increased cystine content in polymorphonuclear leukocytes. * **Treatment:** **Cysteamine** (depletes lysosomal cystine).

Q: Acute and recurrent pancreatitis is reported to occur in which of the following conditions?

Methylmalonic acidemia. **Explanation:** **Methylmalonic Acidemia (MMA)** is an autosomal recessive organic acidemia caused by a deficiency of the enzyme methylmalonyl-CoA mutase or its cofactor, adenosylcobalamin. While primarily presenting with metabolic acidosis, hyperammonemia, and ketonuria, **acute and recurrent pancreatitis** is a well-recognized and serious complication of MMA. The exact pathophysiology is not fully elucidated, but it is believed that the accumulation of toxic metabolites (like methylmalonic acid and propionyl-CoA) causes mitochondrial dysfunction and oxidative stress within pancreatic acinar cells, leading to autodigestion and inflammation. **Analysis of Incorrect Options:** * **Homocystinuria:** Characterized by ectopia lentis, marfanoid habitus, and a high risk of **thromboembolism**. It is not typically associated with pancreatitis. * **Maple Syrup Urine Disorder (MSUD):** Caused by a deficiency in branched-chain alpha-keto acid dehydrogenase. It presents with a "maple syrup" odor in urine and neurological deterioration (seizures, encephalopathy) but not pancreatitis. * **Tyrosinemia (Type I):** Primarily affects the liver and kidneys, leading to **liver failure, hepatocellular carcinoma**, and renal Fanconi syndrome. Pancreatitis is not a feature. **Clinical Pearls for NEET-PG:** * **Organic Acidemias & Pancreatitis:** Both **Methylmalonic acidemia** and **Propionic acidemia** are high-yield causes of recurrent pancreatitis in the pediatric population. * **Triad of MMA:** Recurrent vomiting, metabolic acidosis, and hyperammonemia. * **Management Hint:** Long-term management involves a protein-restricted diet and L-carnitine supplementation; some cases of MMA respond to Vitamin B12 (Hydroxycobalamin).

Q: What is the karyotype of testicular feminisation?

46 XY. **Explanation:** **Testicular Feminization Syndrome**, now more commonly known as **Androgen Insensitivity Syndrome (AIS)**, is a condition where an individual is genetically male but has a female phenotype. 1. **Why 46, XY is correct:** The underlying pathology is a mutation in the **Androgen Receptor (AR) gene** located on the X chromosome. These individuals have normal male chromosomes (**46, XY**) and functional testes (usually undescended). The testes produce normal or high levels of testosterone; however, because the peripheral receptors are unresponsive, the body defaults to a female external phenotype. Since Anti-Müllerian Hormone (AMH) is still produced by the Sertoli cells, internal female structures (uterus, fallopian tubes) are absent. 2. **Why the other options are incorrect:** * **45, XO (Turner Syndrome):** Characterized by a female phenotype, short stature, webbed neck, and streak ovaries. * **47, XXY (Klinefelter Syndrome):** Characterized by a male phenotype, small firm testes, gynecomastia, and infertility. * **46, XXY:** This is a chromosomal abnormality (aneuploidy) similar to Klinefelter, but 47, XXY is the classic representation. **High-Yield Clinical Pearls for NEET-PG:** * **Presentation:** A phenotypically "normal" female presenting with **primary amenorrhea**, a short/blind-ending vagina, and absent pubic/axillary hair (due to androgen resistance). * **Biochemical Profile:** High Testosterone, High LH (due to lack of negative feedback), and normal to high Estrogen (via peripheral aromatization). * **Management:** Gonadectomy is performed after puberty to prevent **Gonadoblastoma** (malignancy risk in undescended testes). * **Key Differentiator:** Unlike Müllerian Agenesis (46, XX), AIS patients have **low/absent pubic hair** and **XY karyotype**.

Q: All of the following clinical features are seen in acute intermittent porphyria, EXCEPT?

Photosensitivity. Explanation: Acute Intermittent Porphyria (AIP) is an autosomal dominant metabolic disorder caused by a deficiency of the enzyme **Porphobilinogen (PBG) deaminase**. This deficiency leads to the accumulation of porphyrin precursors, specifically **delta-aminolevulinic acid (ALA)** and **porphobilinogen (PBG)**. **Why Photosensitivity is the Correct Answer:** Photosensitivity is **absent** in AIP [2]. Cutaneous manifestations (blisters, scarring) occur in porphyrias where there is an accumulation of *porphyrins* (which are photo-excitable). In AIP, the metabolic block occurs early in the heme synthesis pathway, leading to the accumulation of *porphyrin precursors* (ALA and PBG), which are neurotoxic but not photosensitizing. **Analysis of Incorrect Options:** * **Abdominal Pain:** The most common presenting symptom (90% of cases). It is typically severe, poorly localized, and out of proportion to physical findings (neurogenic pain) [1]. * **Seizures:** A common neurological manifestation caused by either direct neurotoxicity of ALA or secondary to hyponatremia (often due to SIADH) [1]. * **Constipation:** A result of autonomic neuropathy affecting the enteric nervous system [1]. Other autonomic features include tachycardia and hypertension. **High-Yield Clinical Pearls for NEET-PG:** * **The "5 Ps" of AIP:** **P**ain (Abdomen), **P**olyneuropathy, **P**sychiatric disturbances, **P**ort-wine colored urine, and **P**recipitated by drugs (e.g., Barbiturates, Sulfonamides) [1]. * **Diagnosis:** Elevated urinary PBG levels during an attack [1]. Urine turns dark/reddish upon standing (oxidation of PBG to porphobilin). * **Management:** Intravenous **Hematin/Heme arginate** (suppresses ALA synthase) and high-dose **Glucose** (carbohydrate loading).

Q: Which of the following is an X-linked mucopolysaccharidosis?

Hunter's syndrome. **Explanation:** The Mucopolysaccharidoses (MPS) are a group of lysosomal storage disorders caused by the deficiency of enzymes required to break down glycosaminoglycans (GAGs). **Correct Answer: B. Hunter’s Syndrome (MPS II)** Hunter’s syndrome is unique among the mucopolysaccharidoses because it is inherited in an **X-linked recessive** pattern. It is caused by a deficiency of the enzyme **Iduronate-2-sulfatase**, leading to the accumulation of dermatan sulfate and heparan sulfate. Clinically, it presents similarly to Hurler’s syndrome but is generally milder and is characteristically distinguished by the **absence of corneal clouding**. **Incorrect Options:** * **A. Hurler’s Syndrome (MPS IH):** This is the most severe form of MPS, caused by a deficiency of **$\alpha$-L-iduronidase**. Unlike Hunter’s, it is inherited in an **Autosomal Recessive (AR)** pattern and features prominent corneal clouding. * **C. Scheie’s Syndrome (MPS IS):** This is a milder variant of MPS I (also AR inheritance). While patients have corneal clouding and valvular heart disease, they often have a normal lifespan and intelligence. * **D. Sanfilippo’s Syndrome (MPS III):** Inherited as an **AR** trait, this disorder is characterized by severe CNS degeneration and mental retardation with relatively mild somatic features. **High-Yield NEET-PG Pearls:** 1. **Mnemonic:** "The **Hunter** needs clear vision to aim at the **X**" (Hunter = **X**-linked; No corneal clouding). 2. All Mucopolysaccharidoses are **Autosomal Recessive EXCEPT Hunter’s Syndrome** (X-linked). 3. **Urinary Screening:** Large amounts of GAGs in the urine are a diagnostic hallmark. 4. **Enzyme Replacement Therapy (ERT):** Available for MPS I (Laronidase) and MPS II (Idursulfase).

Q: Which of the following is NOT a feature of Hermansky-Pudlak syndrome?

Autosomal dominant inheritance. **Explanation:** Hermansky-Pudlak Syndrome (HPS) is a rare multi-system genetic disorder characterized by defects in **lysosome-related organelles (LROs)**, such as melanosomes and platelet dense granules. **1. Why Option A is correct:** Hermansky-Pudlak syndrome is inherited in an **Autosomal Recessive** pattern, not autosomal dominant. It is caused by mutations in several genes (HPS1 through HPS11) that encode proteins involved in the biogenesis of protein complexes required for organelle trafficking. **2. Why other options are incorrect (Features of HPS):** * **Oculocutaneous Albinism (Option B):** Due to defective melanosome formation and distribution, patients present with hypopigmentation of the skin, hair, and eyes, along with visual acuity issues and nystagmus. * **Bleeding Disorder (Option C):** This is a hallmark of HPS caused by a **storage pool deficiency of platelets**. Specifically, there is an absence or marked reduction of **dense granules** (delta granules), leading to impaired platelet aggregation and prolonged bleeding time. * **Pulmonary Fibrosis (Option D):** This is the most serious complication (especially in HPS-1 and HPS-4 subtypes). It results from the accumulation of a ceroid-lipofuscin-like material in alveolar macrophages, leading to restrictive lung disease, typically manifesting in the third or fourth decade of life. **High-Yield Clinical Pearls for NEET-PG:** * **Triad of HPS:** Albinism + Bleeding diathesis + Ceroid lipofuscin lysosomal storage. * **Diagnostic Test:** Electron microscopy of platelets showing **absence of dense bodies**. * **Differential Diagnosis:** **Chediak-Higashi Syndrome** (also features albinism but is distinguished by giant lysosomal granules in neutrophils and frequent pyogenic infections).

Q: Zellweger syndrome is characterized by a defect in which of the following organelles?

Peroxisome. ### Explanation **Correct Option: D. Peroxisome** Zellweger syndrome (also known as cerebrohepatorenal syndrome) is an autosomal recessive disorder caused by mutations in **PEX genes**. These genes encode **peroxins**, which are proteins essential for the assembly and biogenesis of peroxisomes [1]. In this condition, peroxisomes are either absent or non-functional, leading to a failure in the **alpha-oxidation** and **beta-oxidation of Very Long Chain Fatty Acids (VLCFAs)** [2]. This results in the toxic accumulation of VLCFAs and phytanic acid in tissues, particularly the brain and liver [2]. **Why Incorrect Options are Wrong:** * **A. Endoplasmic Reticulum:** The ER is involved in protein folding and lipid synthesis. Defects here lead to conditions like cystic fibrosis (misfolding) or osteogenesis imperfecta, but not Zellweger syndrome. * **B. Lysosome:** Defects in lysosomal enzymes lead to **Lysosomal Storage Diseases** (e.g., Gaucher, Tay-Sachs, or I-cell disease). While both involve organelle dysfunction, lysosomes primarily degrade complex carbohydrates and sphingolipids. * **C. Mitochondria:** Mitochondrial disorders (e.g., MELAS, MERFF) typically involve defects in the electron transport chain or oxidative phosphorylation, leading to lactic acidosis and muscle weakness [2]. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Triad:** Craniofacial dysmorphism (high forehead, wide fontanelles), hepatomegaly, and severe neurological dysfunction (hypotonia, seizures). * **Biochemical Marker:** Elevated levels of **VLCFAs** in the blood is the diagnostic hallmark [2]. * **Prognosis:** It is the most severe of the Peroxisome Biogenesis Disorders (PBD), usually fatal within the first year of life [2]. * **Related Condition:** **Adrenoleukodystrophy (X-linked)** is also a peroxisomal disorder, but it specifically affects the transport of VLCFAs into the peroxisome (ABCD1 mutation), rather than organelle biogenesis.

Q: Conjugated hyperbilirubinemia is seen in which of the following conditions?

Dubin-Johnson syndrome. Hyperbilirubinemia is classified based on whether the elevation is in the **unconjugated (indirect)** or **conjugated (direct)** fraction of bilirubin. ### **Explanation of the Correct Option** **D. Dubin-Johnson syndrome:** This is an autosomal recessive disorder caused by a mutation in the **MRP2 (ABCC2) gene**, which encodes a canalicular multispecific organic anion transporter. While the liver can conjugate bilirubin normally, it cannot transport the conjugated bilirubin into the bile canaliculi. This leads to a reflux of conjugated bilirubin into the blood. A classic clinical feature is a **grossly black liver** due to the accumulation of epinephrine metabolites in lysosomes. ### **Explanation of Incorrect Options** * **A. Gilbert syndrome:** Caused by a mild reduction in the activity of the enzyme **UDP-glucuronosyltransferase (UGT1A1)**. It results in mild, fluctuating **unconjugated** hyperbilirubinemia, often triggered by stress or fasting. * **B. Crigler-Najjar syndrome:** Caused by a severe deficiency (Type II) or total absence (Type I) of **UGT1A1**. This leads to severe **unconjugated** hyperbilirubinemia, with Type I often resulting in kernicterus. * **C. Breast milk jaundice:** Occurs due to substances in breast milk (like beta-glucuronidase or pregnane-3-alpha, 20-beta diol) that inhibit UGT1A1 or increase enterohepatic circulation, leading to **unconjugated** hyperbilirubinemia. ### **NEET-PG High-Yield Pearls** * **Conjugated Hyperbilirubinemia:** Think **Dubin-Johnson** (Black liver, normal gallbladder visualization) and **Rotor Syndrome** (No black liver, gallbladder not visualized on oral cholecystography). * **Urinary Coproporphyrin:** In Dubin-Johnson, total urinary coproporphyrin is normal, but **>80% is Coproporphyrin I** (vs. the normal Coproporphyrin III). * **Mnemonic:** "Dubin-Johnson has a **D**efect in **D**irect bilirubin secretion and a **D**ark liver."

Question 1

Which of the following is an X-linked dominant disorder?

Accepted Answer

Vitamin D resistant rickets

Answer

Fragile X syndrome

Answer

Duchenne muscular dystrophy

Answer

Myotonic dystrophy

Question 2

A clinical study of adults with a body mass index of at least 30 is undertaken. About 8% of these individuals do not have hyperphagia but are found to have normal levels of leptin and ghrelin, along with a diminished basal metabolic rate. A mutation in which of the following genes is most likely present in these individuals?

Accepted Answer

MC4R

Answer

OB-R

Answer

OB

Answer

POMC

Question 3

Which of the following statements about cystinosis is incorrect?

Accepted Answer

Cystine stones are commonly found in the urine.

Answer

Corneal crystals are a characteristic feature.

Answer

Fanconi syndrome can occur.

Answer

Patients may present with white, blond hair and photophobia.

Question 4

Acute and recurrent pancreatitis is reported to occur in which of the following conditions?

Accepted Answer

Methylmalonic acidemia

Answer

Homocystinuria

Answer

Maple syrup urine disorder

Answer

Tyrosinemia

Question 5

What is the karyotype of testicular feminisation?

Accepted Answer

46 XY

Answer

45 XO

Answer

47 XXY

Answer

46 XXY

Question 6

All of the following clinical features are seen in acute intermittent porphyria, EXCEPT?

Accepted Answer

Photosensitivity

Answer

Abdominal pain

Answer

Seizures

Answer

Constipation

Question 7

Which of the following is an X-linked mucopolysaccharidosis?

Accepted Answer

Hunter's syndrome

Answer

Hurler's syndrome

Answer

Scheie's syndrome

Answer

Sanfilippo's syndrome

Question 8

Which of the following is NOT a feature of Hermansky-Pudlak syndrome?

Accepted Answer

Autosomal dominant inheritance

Answer

Oculocutaneous albinism

Answer

Bleeding disorder

Answer

Pulmonary fibrosis

Question 9

Zellweger syndrome is characterized by a defect in which of the following organelles?

Accepted Answer

Peroxisome

Answer

Endoplasmic reticulum

Answer

Lysosome

Answer

Mitochondria

Question 10

Conjugated hyperbilirubinemia is seen in which of the following conditions?

Accepted Answer

Dubin-Johnson syndrome

Answer

Gilbert syndrome

Answer

Crigler-Najjar syndrome

Answer

Breast milk jaundice

Genetic Disorders and Biochemical Pathology — MCQs

Genetic Disorders and Biochemical Pathology — MCQs

On this page

Practice by Chapter

Want unlimited practice?