Biochemistry
1 questionsHow many isoenzymes does lactate dehydrogenase (LDH) have?
NEET-PG 2015 - Biochemistry NEET-PG Practice Questions and MCQs
Question 531: How many isoenzymes does lactate dehydrogenase (LDH) have?
- A. 5, based on H and M polypeptide subunits (Correct Answer)
- B. 7, based on H and M polypeptide subunits
- C. 9, based on H and M polypeptide subunits
- D. 3, based on H and M polypeptide subunits
Explanation: **5, based on H and M polypeptide subunits** - **Lactate dehydrogenase (LDH)** is a tetrameric enzyme, meaning it is composed of four polypeptide subunits. - These subunits can be either **H (heart)** type or **M (muscle)** type, leading to five distinct isoenzymes (**LDH-1, LDH-2, LDH-3, LDH-4, LDH-5**) based on their combinations (HHHH, HHHM, HHMM, HMMM, MMMM). *7, based on H and M polypeptide subunits* - While LDH is composed of two types of subunits, H and M, the possible combinations of these four subunits result in **five distinct isoenzymes**, not seven. - Seven isoenzymes are not a recognized number for LDH. *9, based on H and M polypeptide subunits* - The combination of two types of subunits in a tetrameric structure cannot yield nine unique isoenzymes. - This number is incorrect and not supported by the biochemistry of LDH. *3, based on H and M polypeptide subunits* - Three isoenzymes would imply either fewer than four subunits or a more restricted combination, which is not the case for LDH's tetrameric structure with H and M subunits. - This number is insufficient to account for all possible combinations.
Pathology
1 questionsWhat is the first cellular response observed after a sharp nerve cut?
NEET-PG 2015 - Pathology NEET-PG Practice Questions and MCQs
Question 531: What is the first cellular response observed after a sharp nerve cut?
- A. Chromatolysis (Correct Answer)
- B. Polymorphic arrangement
- C. Increased protein synthesis
- D. Macrophage activation
Explanation: ***Chromatolysis*** - **Chromatolysis** is the dissolution of the Nissl bodies (rough endoplasmic reticulum) in the neuron cell body following axonal injury, which is the **first observable cellular response** [1]. - This process is a preparatory step for neuronal regeneration, indicating the cell's attempt to repair the damaged axon [1]. *Polymorphic arrangement* - This term is not typically used to describe an immediate cellular response to a nerve cut; it might refer to diverse cell shapes or arrangements in different contexts but is not a recognized initial post-injury event. - The neuron's immediate response involves changes within the cell body, not a re-arrangement of its cellular structure with other cells. *Increased protein synthesis* - While increased protein synthesis does occur during neuronal repair and regeneration, it is a consequence of chromatolysis and part of a later, more sustained response, not the very first visible cellular change [1]. - **Chromatolysis precedes** and facilitates the subsequent increase in protein synthesis necessary for axonal regrowth [1]. *Macrophage activation* - **Macrophage activation** is a crucial part of the inflammatory response and debris clearance following nerve injury, but it is not the *first cellular response* of the neuron itself [2]. - Macrophages migrate to the site of injury hours to days after the initial insult, whereas chromatolysis begins within the neuron's cell body much earlier [2]. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Peripheral Nerves and Skeletal Muscles, pp. 1254-1256. [2] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Manifestations Of Central And Peripheral Nervous System Disease, pp. 697-698.
Pharmacology
8 questionsWhich of the following is a primary use of Levamisole?
Which of the following is not excreted by the kidney?
Thymidine is responsible for resistance to which antibiotic ?
Emtricitabine is a/an:
Which antitubercular drug makes the patient non-infective the earliest?
In patients undergoing INH therapy, which group is least likely to develop neuropathy?
Among the following statins, which has the longest half-life?
Which of the following is a characteristic of simvastatin?
NEET-PG 2015 - Pharmacology NEET-PG Practice Questions and MCQs
Question 531: Which of the following is a primary use of Levamisole?
- A. Immunostimulant
- B. Antihelminthic (Correct Answer)
- C. None of the options
- D. Immunomodulator
Explanation: ***Antihelminthic*** - Levamisole is **primarily classified as an antihelminthic drug**, used to treat parasitic worm infections. - It acts as a **nicotinic receptor agonist** in nematodes, causing spastic paralysis of the worms, leading to their expulsion. - It was historically used in humans for treating ascariasis and hookworm infections, and is still used in **veterinary medicine** for deworming livestock. - This is its **primary pharmacological classification** in standard medical textbooks. *Immunomodulator* - Levamisole does have **immunomodulatory properties** that were discovered secondary to its antihelminthic use. - It was used as **adjuvant therapy in colon cancer** (with 5-FU) to enhance immune response. - However, this is a **secondary use**, not its primary classification, and has been largely discontinued due to severe side effects like agranulocytosis. *Immunostimulant* - While levamisole can stimulate certain aspects of cell-mediated immunity, this overlaps with its immunomodulatory effects. - This is **not its primary pharmacological classification** - it remains primarily an antihelminthic agent. *None of the options* - This is incorrect because **antihelminthic** is clearly the primary and correct classification of levamisole in pharmacology. - Its antihelminthic action was its original and primary therapeutic application.
Question 532: Which of the following is not excreted by the kidney?
- A. Ciprofloxacin
- B. Ofloxacin
- C. Moxifloxacin (Correct Answer)
- D. Levofloxacin
Explanation: ***Moxifloxacin*** - Moxifloxacin is primarily metabolized in the **liver** and excreted through bile and feces. - This characteristic makes it a suitable choice for patients with significant **renal impairment** as dose adjustments are generally not required. *Ciprofloxacin* - Ciprofloxacin is predominantly excreted by the **kidneys** through both glomerular filtration and tubular secretion. - Dose adjustments are crucial in patients with **renal dysfunction** to prevent accumulation and toxicity. *Ofloxacin* - Ofloxacin is largely excreted unchanged in the **urine**, making renal excretion its primary elimination pathway. - **Dose reduction** is necessary for patients with impaired renal function. *Levofloxacin* - Levofloxacin is primarily eliminated via **renal excretion**, with a significant portion appearing in the urine as unmetabolized drug. - Patients with **kidney disease** require appropriate dose adjustments.
Question 533: Thymidine is responsible for resistance to which antibiotic ?
- A. Erythromycin
- B. Sulfonamide (Correct Answer)
- C. Tetracycline
- D. Nitrofurantoin
Explanation: ***Sulfonamide*** - **Thymidine** can contribute to **sulfonamide resistance** because sulfonamides interfere with **folate metabolism** and the subsequent synthesis of purines and pyrimidines, including thymidine. - An excess of thymidine can bypass the metabolic block caused by sulfonamides, allowing bacteria to continue DNA synthesis and grow. *Erythromycin* - **Erythromycin** resistance is primarily mediated by **methylation of ribosomal RNA**, which prevents the antibiotic from binding to the 50S ribosomal subunit. - It does not directly involve thymidine or the folate synthesis pathway. *Tetracycline* - Resistance to **tetracyclines** is commonly due to **efflux pumps** that actively pump the drug out of the bacterial cell or **ribosomal protection proteins** that prevent tetracycline binding. - Thymidine production or metabolism is not a mechanism of tetracycline resistance. *Nitrofurantoin* - **Nitrofurantoin** resistance typically involves **mutations** in bacterial enzymes (like **nitrofuran reductase**) that are responsible for activating the drug into its active form. - These mutations prevent the drug from becoming bactericidal, and thymidine does not play a role in this mechanism.
Question 534: Emtricitabine is a/an:
- A. Alkylating agent
- B. Mitotic inhibitor
- C. Nucleoside reverse transcriptase inhibitor (NRTI) (Correct Answer)
- D. None of the options
Explanation: ***Nucleoside reverse transcriptase inhibitor (NRTI)*** - **Emtricitabine** is a synthetic nucleoside analog that inhibits the activity of HIV-1 **reverse transcriptase**, an enzyme essential for viral replication. - It works by being phosphorylated to its active triphosphate form, which then competes with natural deoxycytidine triphosphate for incorporation into the viral DNA, leading to **chain termination**. *Alkylating agent* - **Alkylating agents** are a class of antineoplastic drugs that work by adding an alkyl group to DNA, forming a covalent bond that interferes with DNA replication and transcription. - They are primarily used in **cancer chemotherapy**, not as antiviral agents for HIV. *Mitotic inhibitor* - **Mitotic inhibitors** are drugs that interfere with cell division (mitosis) by targeting microtubules, either inhibiting their polymerization or depolymerization. - These agents are also used in **cancer treatment** to prevent rapidly dividing cells from completing mitosis. *None of the options* - This option is incorrect because **emtricitabine** clearly belongs to the class of **nucleoside reverse transcriptase inhibitors**.
Question 535: Which antitubercular drug makes the patient non-infective the earliest?
- A. Ethambutol
- B. Pyrazinamide
- C. Isoniazid (INH) (Correct Answer)
- D. Rifampin
Explanation: ***Isoniazid (INH)*** - **Isoniazid** renders TB patients **non-infectious the fastest**, typically within **2-3 days** of starting treatment - It has the most **rapid bactericidal effect** against actively multiplying extracellular **Mycobacterium tuberculosis**, which are the primary organisms responsible for transmission - INH works by inhibiting **mycolic acid synthesis**, disrupting the bacterial cell wall of rapidly dividing bacilli - This makes it the most critical drug for **early infection control** and reducing community transmission *Rifampin* - While **rifampin** is highly bactericidal and has excellent sterilizing activity, it takes **slightly longer** than INH to render patients non-infectious - Rifampin is particularly effective against **semi-dormant organisms** and intracellular bacilli - It is the most important drug for **preventing relapse** and shortening treatment duration, but INH acts faster in reducing infectivity *Ethambutol* - **Ethambutol** is primarily **bacteriostatic**, inhibiting arabinosyl transferase and interfering with cell wall synthesis - Its main role is to **prevent emergence of drug resistance** rather than rapidly reducing bacterial load - Has minimal impact on early infectivity reduction *Pyrazinamide* - **Pyrazinamide** is most effective against **semi-dormant bacilli** within macrophages and in acidic environments - Its **sterilizing activity** helps shorten overall treatment duration but does not contribute significantly to rapid reduction in infectivity - Works slowly and is not bactericidal against actively multiplying extracellular organisms
Question 536: In patients undergoing INH therapy, which group is least likely to develop neuropathy?
- A. Having malnutrition
- B. Alcoholics
- C. Fast acetylators (Correct Answer)
- D. Vitamin B complex deficiency
Explanation: ***Fast acetylators*** - **Fast acetylators** metabolize INH more quickly, leading to lower systemic drug levels and thus a reduced risk of adverse effects like neuropathy. - Neuropathy associated with INH is primarily due to **pyridoxine (vitamin B6) depletion**, which is less pronounced if the drug is rapidly cleared. *Having malnutrition* - **Malnutrition** often involves deficiencies in essential vitamins, including vitamin B6, which is crucial for preventing INH-induced neuropathy. - Patients with poor nutritional status are at a **higher risk** of developing neuropathy during INH therapy due to pre-existing vitamin B6 depletion. *Alcoholics* - **Alcoholism** is strongly associated with deficiencies in various B vitamins, particularly **pyridoxine (vitamin B6)**, due to poor diet and impaired absorption. - This pre-existing deficiency makes alcoholics **highly susceptible** to INH-induced neuropathy. *Vitamin B complex deficiency* - A **deficiency in vitamin B complex**, especially pyridoxine (B6), is a known risk factor for INH-induced neuropathy. - Isoniazid interferes with **pyridoxine metabolism**, and those with pre-existing deficiency are more vulnerable to this adverse effect.
Question 537: Among the following statins, which has the longest half-life?
- A. Pravastatin
- B. Simvastatin
- C. Lovastatin
- D. Rosuvastatin (Correct Answer)
Explanation: **Rosuvastatin** - **Rosuvastatin** has the longest half-life among the commonly used statins, approximately **19 hours**, allowing for consistent lipid-lowering effects. - Its prolonged presence in the body contributes to its effectiveness in reducing **LDL-C** at lower doses. *Pravastatin* - **Pravastatin** has a relatively short half-life of about **1.8 hours**, requiring daily dosing to maintain therapeutic concentrations. - Its hydrophilic nature means it is less likely to penetrate non-hepatic tissues, potentially reducing extrahepatic side effects. *Simvastatin* - **Simvastatin** has a short half-life of about **3 hours**, necessitating daily administration. - It is a **prodrug** that requires hepatic activation to its active beta-hydroxy acid form. *Lovastatin* - **Lovastatin** also has a short half-life, around **3 hours**, and is a **prodrug** like simvastatin. - It is often recommended to be taken in the evening due to the diurnal rhythm of cholesterol synthesis.
Question 538: Which of the following is a characteristic of simvastatin?
- A. Specific CYP3A4 substrate with high interaction potential
- B. Derived from fungal metabolite (Correct Answer)
- C. Prodrug requiring hepatic activation
- D. Short half-life requiring evening dosing
Explanation: ***Derived from fungal metabolite*** - **Simvastatin** and lovastatin are **naturally-derived statins** obtained from **fungal metabolites** (*Aspergillus terreus*), distinguishing them from synthetic statins like atorvastatin, rosuvastatin, and pravastatin [2]. - This is the **most distinguishing characteristic** for classification purposes, as it represents the drug's origin and places it in a specific subclass of HMG-CoA reductase inhibitors. - The discovery of fungal-derived statins led to the development of the entire statin drug class. *Prodrug requiring hepatic activation* - While **simvastatin** is a **lactone prodrug** requiring hepatic hydrolysis to its active beta-hydroxy acid form, this is a pharmacokinetic property shared with lovastatin [1]. - This is a characteristic but not the most distinguishing feature for classification. *Specific CYP3A4 substrate with high interaction potential* - **Simvastatin** is extensively metabolized by **CYP3A4**, leading to significant drug-drug interactions with CYP3A4 inhibitors (e.g., ketoconazole, erythromycin, grapefruit juice). - While clinically important, many drugs are CYP3A4 substrates, making this less distinctive as a defining characteristic. *Short half-life requiring evening dosing* - **Simvastatin** has a **short half-life** (2-3 hours) and is preferably administered in the evening because cholesterol synthesis is highest at night. - This is a dosing consideration based on pharmacokinetics rather than a fundamental distinguishing characteristic of the drug's identity.