Membrane Disorders and Diseases — MCQs

10 questions

Which of the following bacteria produces a toxin that inhibits protein synthesis?

Pertussis toxin acts by all of the following mechanisms except ?

An 8 year old boy complains of increasing muscle weakness. On examination, his calves are bulky and show muscle tightening. His serum creatine kinase levels are increasing with age. Which of the following is the most likely diagnosis?

Which of the following is a genetic disorder that does not primarily affect ion channels?

Which of the following statements regarding hereditary spherocytosis is false?

Defect in myasthenia gravis is at the level of?

An infant presenting with failure to thrive, hypertension, metabolic acidosis, and hyperkalemia is most likely suffering from which condition?

Which of the following conditions is characterized by the presence of hyaline deposits in alveolar walls?

Increased susceptibility to N. meningitidis infections is associated with deficiency of which complement component:

Q10

Pink disease is seen due to

Membrane Disorders and Diseases Indian Medical PG Practice Questions and MCQs

Question 1: Which of the following bacteria produces a toxin that inhibits protein synthesis?

A. Corynebacterium diphtheriae (Correct Answer)
B. Bacillus anthracis
C. Staphylococcus aureus
D. Vibrio cholerae

Explanation: ***Corynebacterium diphtheriae*** - This bacterium produces the **diphtheria toxin**, which is an **A-B toxin**. - The **A subunit** of the toxin inhibits **protein synthesis** by inactivating elongation factor-2 (EF-2), leading to cell death. *Bacillus anthracis* - Produces **anthrax toxin**, which consists of three components: protective antigen, edema factor, and lethal factor. - The **lethal factor** cleaves specific protein kinases, while the **edema factor** acts as an adenylate cyclase, disturbing cell water balance, but neither directly inhibits protein synthesis in the same manner as diphtheria toxin. *Staphylococcus aureus* - Produces numerous toxins, including **superantigens** (like toxic shock syndrome toxin-1) and **exfoliative toxins**, which cause scaly skin, and **alpha-toxin**, which forms pores in cell membranes. - These toxins have diverse mechanisms of action, but none primarily inhibit **protein synthesis**. *Vibrio cholerae* - Produces **cholera toxin**, an **A-B toxin**, which acts on intestinal cells. - The **A subunit** irreversibly activates **adenylate cyclase**, leading to increased cyclic AMP (cAMP) levels and massive efflux of water and electrolytes, causing severe diarrhea, but it does not inhibit protein synthesis.

Question 2: Pertussis toxin acts by all of the following mechanisms except ?

A. Increase cyclic AMP
B. Acts through G alpha subunit
C. ADP ribosylation of proteins associated with receptors
D. Increased calcium release from sarcoplasmic reticulum (Correct Answer)

Explanation: ***Increased calcium release from sarcoplasmic reticulum*** - Pertussis toxin does NOT directly cause increased calcium release from the sarcoplasmic reticulum. - While increased cAMP can have downstream effects on calcium signaling, **direct calcium release from SR is not a characteristic mechanism** of pertussis toxin action. - This is the exception among the listed mechanisms. *Acts through G alpha subunit* - Pertussis toxin primarily acts by **ADP-ribosylating** the αi subunit of **G proteins** (Gαi), effectively inhibiting its function. - This inhibition leads to the **inactivation of Gi proteins**, preventing them from inhibiting adenylate cyclase. - This is the PRIMARY mechanism of pertussis toxin. *Increase cyclic AMP* - The inhibition of Gi proteins by pertussis toxin leads to **constitutive activation of adenylate cyclase**. - This results in a **sustained increase in intracellular cyclic AMP (cAMP)** levels. - This is a well-established consequence of pertussis toxin action. *ADP ribosylation of proteins associated with receptors* - Pertussis toxin specifically acts as an **ADP-ribosyltransferase**, transferring an ADP-ribose group from NAD+ to target proteins. - This modification occurs on the **alpha-inhibitory subunit (Giα)** of heterotrimeric G proteins, which are associated with various cell surface receptors. - This is the molecular mechanism by which pertussis toxin exerts its effects.

Question 3: An 8 year old boy complains of increasing muscle weakness. On examination, his calves are bulky and show muscle tightening. His serum creatine kinase levels are increasing with age. Which of the following is the most likely diagnosis?

A. Congenital myopathy
B. Dystrophin deficiency (Correct Answer)
C. Myelin deficiency
D. Hereditary sensorimotor neuropathy
E. Becker muscular dystrophy

Explanation: ***Dystrophin deficiency*** - The presentation of an 8-year-old boy with **increasing muscle weakness**, **bulky calves** (pseudohypertrophy), and **elevated creatine kinase** is highly characteristic of Duchenne muscular dystrophy, which is caused by a dystrophin deficiency. - The muscle tightening and progressive increase in **creatine kinase** over time further support this diagnosis, as muscle breakdown leads to enzyme release. - Duchenne typically presents between **3-5 years** with progressive weakness, wheelchair dependence by early teens, and very high CK levels (10-100x normal). *Becker muscular dystrophy* - While also caused by **dystrophin deficiency** (partial rather than complete), Becker muscular dystrophy has a **later onset** (usually age 10-15 years) and **milder, slower progression**. - The age of 8 years with significant symptoms and the rapid progression described are more consistent with **Duchenne** than Becker. *Congenital myopathy* - While hereditary muscle disorders, congenital myopathies typically present at birth or in early infancy with **generalized hypotonia** and **muscle weakness**, often static or slowly progressive. - They usually do not exhibit the prominent calf pseudohypertrophy seen in Duchenne muscular dystrophy. *Myelin deficiency* - Myelin deficiency disorders primarily affect the **nervous system**, leading to issues with nerve signal transmission rather than direct muscle weakness and pseudohypertrophy. - Symptoms would typically include **neurological deficits** such as sensory loss, ataxia, or spasticity. *Hereditary sensorimotor neuropathy* - These neuropathies (e.g., Charcot-Marie-Tooth disease) involve both **sensory and motor nerves**, leading to muscle weakness, atrophy, and sensory loss, often in the distal limbs. - They do not typically cause **calf pseudohypertrophy** or progressively high creatine kinase levels as seen in primary muscle diseases.

Question 4: Which of the following is a genetic disorder that does not primarily affect ion channels?

A. Cystic fibrosis
B. Liddle's syndrome
C. Hypokalemic periodic paralysis
D. Tay-Sachs disease (Correct Answer)

Explanation: ***Cystic fibrosis*** - Primarily caused by mutations in the **CFTR gene**, which encodes a **chloride channel**, but it is not classified as a classic channelopathy. - The disease mainly affects **mucus production** rather than direct dysfunction of ion channels in the traditional sense. [1] *Hypokalemic periodic paralysis* - This condition is directly related to **ion channel dysfunction**, specifically affecting **sodium channels** in muscle cells. - It causes episodic **muscle weakness** and hypokalemia due to improper ion transport. *Liddle's syndrome* - A genetic disorder resulting from mutations affecting **epithelial sodium channels**, leading to **hypertension**. - It exemplifies classic channelopathy by causing dysregulation of sodium reabsorption in the kidneys. *Tay-sach's disease* - A ***gangliosidosis*** caused by a deficiency in the enzyme **Hexosaminidase A**, rather than ion channel dysfunction. - It results in the accumulation of **GM2 gangliosides** leading to neurological degeneration, not affecting ion channels directly. **References:** [1] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. (Basic Pathology) introduces the student to key general principles of pathology, both as a medical science and as a clinical activity with a vital role in patient care. Part 2 (Disease Mechanisms) provides fundamental knowledge about the cellular and molecular processes involved in diseases, providing the rationale for their treatment. Part 3 (Systematic Pathology) deals in detail with specific diseases, with emphasis on the clinically important aspects., pp. 120-122.

Question 5: Which of the following statements regarding hereditary spherocytosis is false?

A. Anemia
B. It is due to intrinsic defect in RBC membrane skeleton
C. Spherocytes are the characteristic finding on peripheral blood smear
D. Splenomegaly is not a feature of hereditary spherocytosis (Correct Answer)

Explanation: ***Spherocytosis is a pathognomic finding*** - Spherocytosis is not **pathognomic** for hereditary spherocytosis as it can also occur in other conditions like autoimmune hemolytic anemia. - While **spherocytes** are a notable finding in hereditary spherocytosis, they are not exclusive to this disorder. *Anemia* - Anemia is a common result of **hemolysis** in hereditary spherocytosis due to the destruction of spherical red blood cells [1]. - Patients often present with **mild to moderate anemia** [1][2], but this is a characteristic finding, not an exception. *Splenomegaly* - Splenomegaly is frequently observed in hereditary spherocytosis due to **hypertrophy** from increased red blood cell destruction. - This finding is **typical** in cases of hereditary spherocytosis and serves as an important clinical indicator [2]. *It is due to intrinsic defect in RBC membrane skeleton* - Hereditary spherocytosis is primarily caused by an **intrinsic defect in the red blood cell membrane**, particularly affecting proteins like spectrin and ankyrin [1]. - This intrinsic defect leads to the formation of **spherocytes**, which are less deformable and prone to hemolysis [1][2]. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Red Blood Cell and Bleeding Disorders, pp. 640-642. [2] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Blood And Bone Marrow Disease, pp. 597-598.

Question 6: Defect in myasthenia gravis is at the level of?

A. Brain
B. Neuromuscular junction
C. Enzyme
D. Nerve terminal (Correct Answer)

Explanation: ***Nerve terminal*** - Myasthenia gravis is an **autoimmune disease** characterized by autoantibodies against proteins at the **neuromuscular junction** [1]. - These antibodies specifically target and destroy **acetylcholine receptors** on the postsynaptic membrane, leading to impaired nerve impulse transmission [1], [2]. *Brain* - The brain is part of the central nervous system, and defects causing myasthenia gravis are not located there. - While brain pathologies can cause neurological symptoms, they do not directly lead to the specific muscle weakness seen in myasthenia gravis. *Neuromuscular junction* - This is the synaptic connection between the terminal end of a motor nerve and a muscle fiber. - While the *effect* of the defect is seen at the neuromuscular junction (reduced acetylcholine receptors), the primary *defect* (antibody production) originates from an immune response, and the actual damage occurs to the **postsynaptic membrane**, not the entire junction structurally [2]. *Enzyme* - Myasthenia gravis is not caused by an enzyme deficiency or defect. - While enzymes like **acetylcholinesterase** play a role in acetylcholine metabolism at the neuromuscular junction, their malfunction is not the primary mechanism of myasthenia gravis.

Question 7: An infant presenting with failure to thrive, hypertension, metabolic acidosis, and hyperkalemia is most likely suffering from which condition?

A. Liddle's syndrome
B. Bartter's syndrome
C. Gittelman's syndrome
D. Gordon syndrome (Correct Answer)

Explanation: ***Gordon syndrome*** - **Gordon syndrome** (Pseudohypoaldosteronism Type II) is characterized by **hypertension**, **hyperkalemia**, and **metabolic acidosis**, which perfectly matches the clinical presentation in this infant. - The underlying defect involves abnormal regulation of the **WNK kinase pathway**, leading to increased activity of the thiazide-sensitive Na-Cl cotransporter (NCC) in the distal convoluted tubule, resulting in increased sodium reabsorption and impaired potassium excretion. - This causes **volume expansion** (leading to hypertension), **hyperkalemia** (due to reduced potassium secretion), and **metabolic acidosis** (due to impaired hydrogen ion secretion). *Liddle's syndrome* - This syndrome presents with **hypertension**, **hypokalemia**, and **metabolic alkalosis**, due to increased activity of the epithelial sodium channel (ENaC) in the collecting duct. - The presence of **hyperkalemia** and **metabolic acidosis** rules out Liddle's syndrome. *Bartter's syndrome* - Characterized by **hypokalemia**, **metabolic alkalosis**, and **normal or low blood pressure**, due to impaired reabsorption in the thick ascending limb of the loop of Henle. - The combination of **hypertension**, **hyperkalemia**, and **metabolic acidosis** is completely inconsistent with Bartter's syndrome. *Gitelman's syndrome* - This syndrome typically causes **hypokalemia**, **metabolic alkalosis**, **hypomagnesemia**, and **hypocalciuria**, due to a defect in the thiazide-sensitive NaCl cotransporter in the distal convoluted tubule. - The infant's **hyperkalemia**, **hypertension**, and **metabolic acidosis** are completely inconsistent with Gitelman's syndrome.

Question 8: Which of the following conditions is characterized by the presence of hyaline deposits in alveolar walls?

A. Asthma
B. Hyaline membrane disease (Correct Answer)
C. Chronic bronchitis
D. Interstitial lung disease

Explanation: ***Hyaline membrane disease*** - This condition is pathologically characterized by the presence of **eosinophilic (hyaline) membranes** lining the distal airspaces, which are composed of fibrin, cellular debris, and necrotic cells [1]. - These **hyaline deposits obstruct gas exchange** and are a hallmark of **acute lung injury** in neonates due to surfactant deficiency [2]. *Asthma* - Characterized by **bronchoconstriction**, **mucus plugging**, and **airway inflammation**, but not hyaline deposits in alveolar walls. - Pathologically, there is hyperplasia of goblet cells, hypertrophy of bronchial smooth muscle, and eosinophilic infiltration. *Chronic bronchitis* - Defined by **chronic productive cough** due to hypertrophy of mucous glands and increased mucus production in the bronchi, not alveolar hyaline deposits. - It primarily affects the **large airways** and is associated with chronic inflammation and airway obstruction. *Interstitial lung disease* - Involves inflammation and fibrosis of the **interstitium of the lung**, leading to impaired gas exchange. - While it causes architectural distortion, **hyaline deposits** in the alveolar walls are not a defining pathological feature. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Lung, pp. 679-681. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of Infancy and Childhood, p. 466.

Question 9: Increased susceptibility to N. meningitidis infections is associated with deficiency of which complement component:

A. C1-C4 deficiency
B. C3 deficiency
C. C5-C9 deficiency (Correct Answer)
D. C2 deficiency

Explanation: ***C5-C9 deficiency*** - Deficiencies in **C5-C9 components** impair the formation of the **Membrane Attack Complex (MAC)**, which is crucial for lysing Gram-negative bacteria like **N. meningitidis**. - Patients with MAC deficiencies are at significantly higher risk for recurrent invasive **N. meningitidis** infections. *C1-C4 deficiency* - Deficiencies in **C1-C4 components** primarily affect the **classical complement pathway** and are associated with increased susceptibility to **bacterial infections** and **immune complex diseases** (e.g., SLE). - While these deficiencies compromise opsonization and inflammation, they are not specifically linked to recurrent **N. meningitidis** infections. *C3 deficiency* - **C3 deficiency** is a severe primary immunodeficiency leading to profound defects in complement activation via all pathways, affecting **opsonization** and the formation of the MAC. - This deficiency causes severe recurrent **pyogenic infections** due to encapsulated bacteria but is not as specifically or commonly linked to **N. meningitidis** as deficiencies in the terminal pathway. *C2 deficiency* - **C2 deficiency** is the most common complement deficiency and primarily impacts the **classical pathway**, leading to impaired opsonization and immune complex clearance. - It is often associated with recurrent infections (especially with encapsulated bacteria) and **lupus-like syndromes**, but not specifically increased susceptibility to **N. meningitidis** infections.

Question 10: Pink disease is seen due to

A. Arsenic poisoning
B. Internal resorption
C. Trauma
D. Mercury poisoning (Correct Answer)

Explanation: ***Mercury poisoning*** - **Pink disease**, also known as **acrodynia**, is a historical term for a childhood illness caused by chronic exposure to **mercury** [1]. - Symptoms include a characteristic pink rash on the hands and feet, irritability, photophobia, and hypotonia [1]. *Arsenic poisoning* - **Arsenic poisoning** can lead to symptoms like gastrointestinal distress, skin lesions (hyperkeratosis, melanosis), and neurological effects [2]. - It does not typically cause the characteristic pink rash and other signs associated with acrodynia. *Internal resorption* - **Internal resorption** is a dental condition where the dentin and cementum of a tooth are resorbed from within the pulp chamber or root canal. - This condition is localized to the tooth and is not associated with systemic symptoms or a widespread rash like pink disease. *Trauma* - **Trauma** refers to physical injury and can cause various localized or systemic effects depending on the nature and severity of the injury. - While trauma can lead to discoloration or rashes, it is not a direct cause of the specific syndrome known as pink disease (acrodynia).

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