General Pharmacology — MCQs
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Tachyphylaxis is seen after the use of which of the following drugs?
Therapeutic drug monitoring is done for all of the following drugs except?
Which of the following therapeutic index (T.I.) values represents the greatest safety profile for a drug?
All the following enzymes are involved in the metabolism of xenobiotics except?
Which of the following is an example of physiological antagonism?
General Pharmacology Indian Medical PG Practice Questions and MCQs
Question 1: Tachyphylaxis is seen after the use of which of the following drugs?
- A. Tamoxifen
- B. Ephedrine (Correct Answer)
- C. Morphine
- D. Chlorpromazine
Explanation: **Explanation:** **Tachyphylaxis** is defined as a rapid decrease in response to a drug after repeated administration over a short period. Unlike tolerance, which develops slowly, tachyphylaxis occurs quickly and cannot be overcome by simply increasing the dose. **Why Ephedrine is the Correct Answer:** Ephedrine is a classic example of a drug that exhibits tachyphylaxis. It acts primarily as an **indirect-acting sympathomimetic**, meaning it works by displacing norepinephrine (NE) from storage vesicles in the nerve endings. With repeated, frequent administration, the available stores of NE become depleted. Once the "pool" of neurotransmitters is exhausted, further doses of ephedrine produce little to no pharmacological effect until the nerve endings have time to synthesize and restock NE. **Analysis of Incorrect Options:** * **Tamoxifen (A):** This is a Selective Estrogen Receptor Modulator (SERM). It does not typically show rapid desensitization or tachyphylaxis. * **Morphine (C):** Morphine leads to **Tolerance**, not tachyphylaxis. Tolerance to opioids develops over days or weeks due to receptor downregulation and internalisation, requiring higher doses for the same analgesic effect. * **Chlorpromazine (D):** This is a typical antipsychotic. While patients may develop tolerance to its sedative effects, it does not exhibit the rapid acute waning of effect characteristic of tachyphylaxis. **High-Yield Clinical Pearls for NEET-PG:** * **Common drugs showing Tachyphylaxis:** **T**yramine, **E**phedrine, **A**mphetamine, **N**icotine, **N**itroglycerin, and **D**econgestants (e.g., Xylometazoline). (Mnemonic: **T**ea **E**n**A** **N**i**N**D). * **Mechanism:** Usually due to depletion of endogenous neurotransmitters or rapid receptor phosphorylation/internalization. * **Nitrates:** Tachyphylaxis (often called "nitrate tolerance") occurs due to the depletion of free sulfhydryl (-SH) groups required for NO release; hence, a "nitrate-free interval" is recommended.
Question 2: Therapeutic drug monitoring is done for all of the following drugs except?
- A. Phenytoin
- B. Diclofenac (Correct Answer)
- C. Tacrolimus
- D. Cyclosporine
Explanation: **Explanation:** Therapeutic Drug Monitoring (TDM) is the clinical practice of measuring drug concentrations in the blood to maintain a constant concentration within a specific **therapeutic window**. It is indicated for drugs with a narrow therapeutic index, high inter-individual pharmacokinetic variability, or a direct correlation between plasma levels and clinical effects/toxicity. **Why Diclofenac is the correct answer:** Diclofenac is a Non-Steroidal Anti-Inflammatory Drug (NSAID) with a **wide therapeutic index**. Its clinical effect (analgesia and anti-inflammatory action) can be easily monitored by clinical response (reduction in pain or swelling). Furthermore, there is no established correlation between its plasma concentration and its efficacy or toxicity profile, making TDM unnecessary and impractical. **Analysis of incorrect options:** * **Phenytoin:** An antiepileptic with **zero-order (saturated) kinetics** at therapeutic doses. Small dose increases can lead to disproportionately large increases in plasma levels, causing toxicity (e.g., ataxia, nystagmus). TDM is mandatory. * **Tacrolimus & Cyclosporine:** Both are **calcineurin inhibitors** (immunosuppressants) used in organ transplants. They have a very narrow therapeutic window; low levels lead to graft rejection, while high levels cause significant nephrotoxicity and neurotoxicity. TDM is standard of care for these agents. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for TDM drugs:** "**L**ithium, **I**mmunosuppressants (Cyclosporine/Tacrolimus), **T**ricyclic Antidepressants, **A**nti-epileptics (Phenytoin/Valproate), **D**igoxin, **A**minoglycosides, **T**heophylline" (**LITA DAT**). * **Drugs NOT requiring TDM:** Drugs with easily measurable physiological markers (e.g., Warfarin via PT/INR, Antihypertensives via Blood Pressure, Insulin via Blood Glucose). * **Phenytoin** is a classic "TDM favorite" in exams due to its non-linear pharmacokinetics.
Question 3: Which of the following therapeutic index (T.I.) values represents the greatest safety profile for a drug?
- A. 100
- B. 1,000 (Correct Answer)
- C. 500
- D. 2
Explanation: ### Explanation **Concept Overview** The **Therapeutic Index (T.I.)** is a quantitative measurement of a drug's relative safety [1]. It is defined as the ratio of the dose that produces toxicity to the dose that produces a clinically desired effective response. Mathematically, it is expressed as: **T.I. = TD₅₀ / ED₅₀** (or LD₅₀ / ED₅₀ in animal studies) [1] * **TD₅₀:** Toxic dose in 50% of the population. * **ED₅₀:** Effective dose in 50% of the population. **Why Option B is Correct** The safety of a drug is **directly proportional** to its Therapeutic Index [1]. A higher T.I. indicates a wider "margin of safety," meaning there is a large gap between the dose required for efficacy and the dose that causes toxicity [2]. Among the given options, **1,000** is the highest value, representing the widest margin and, therefore, the greatest safety profile. **Analysis of Incorrect Options** * **Options A (100) and C (500):** While these values represent relatively safe drugs, they are less safe than a drug with a T.I. of 1,000. * **Option D (2):** This represents a **Narrow Therapeutic Index (NTI)** drug. Such drugs are inherently risky because a small increase in dose or a minor change in blood concentration can lead to severe toxicity. **NEET-PG High-Yield Pearls** * **Narrow Therapeutic Index Drugs (Mnemonic: "W-A-R-F-I-N-G-L-E-T"):** **W**arfarin, **A**minoglycosides/Amiodarone, **R**ifampicin, **F**enytoin (Phenytoin), **I**nsulin, **N**eostigmine, **G**lycosides (Digoxin), **L**ithium, **E**thosuximide/Enalapril, **T**heophylline/Tricyclic Antidepressants. * **Clinical Monitoring:** Drugs with a low T.I. often require **Therapeutic Drug Monitoring (TDM)** to ensure safety and efficacy [3]. * **Penicillin** is a classic example of a drug with a very high T.I., making it remarkably safe even at high doses.
Question 4: All the following enzymes are involved in the metabolism of xenobiotics except?
- A. Hydroxylase
- B. Cytochrome oxidase (Correct Answer)
- C. Cytochrome P450
- D. Methylase
Explanation: **Explanation:** The metabolism of xenobiotics (foreign compounds) occurs primarily in the liver through Phase I (Functionalization) and Phase II (Conjugation) reactions. **Why Cytochrome Oxidase is the correct answer:** Cytochrome oxidase (also known as **Cytochrome c oxidase** or Complex IV) is a key enzyme in the **Electron Transport Chain** located in the inner mitochondrial membrane. Its primary role is cellular respiration—transferring electrons to oxygen to form water—rather than the detoxification of drugs. While it deals with oxygen, it is not involved in the metabolic transformation of xenobiotics. **Analysis of other options:** * **Cytochrome P450 (Option C):** This is the most important superfamily of enzymes involved in Phase I metabolism. They are hemoproteins located in the smooth endoplasmic reticulum that catalyze oxidative reactions for the majority of clinical drugs. * **Hydroxylase (Option A):** Hydroboxylation is a classic Phase I reaction. Many Cytochrome P450 enzymes function as mixed-function oxidases or hydroxylases, adding an –OH group to the substrate to make it more polar. * **Methylase (Option D):** Methylation is a specific type of Phase II conjugation reaction (e.g., COMT acting on catecholamines). It involves the transfer of a methyl group to the xenobiotic to alter its activity or facilitate excretion. **Clinical Pearls for NEET-PG:** * **Phase I reactions:** Oxidation (most common), Reduction, and Hydrolysis. * **Phase II reactions:** Glucuronidation (most common), Acetylation, Methylation, Sulfation, and Glutathione conjugation. * **Microsomal vs. Non-microsomal:** Cytochrome P450 and Glucuronosyltransferase are **microsomal** (located in the SER). Most other Phase II enzymes and non-P450 oxidative enzymes (like Alcohol Dehydrogenase) are **non-microsomal** (cytosolic or mitochondrial). * **Inducer vs. Inhibitor:** Remember that CYP3A4 is the most common isoform involved in drug metabolism. Drugs like Rifampicin induce these enzymes, while Ketoconazole inhibits them.
Question 5: Which of the following is an example of physiological antagonism?
- A. Heparin–Protamine
- B. Prostacycline–Thromboxane (Correct Answer)
- C. Adrenaline–Phenoxybenzamine
- D. Physostigmine–Acetylcholine
Explanation: ### Explanation **Physiological (Functional) Antagonism** occurs when two drugs act on **different receptors** or through different mechanisms, but produce **opposing physiological effects** on the same biological system. **1. Why Option B is Correct:** * **Prostacyclin (PGI2)** acts on IP receptors to cause vasodilation and inhibition of platelet aggregation. * **Thromboxane A2 (TXA2)** acts on TP receptors to cause vasoconstriction and promotion of platelet aggregation. * Since they produce opposite effects using distinct pathways/receptors, they are classic examples of physiological antagonists. **2. Analysis of Incorrect Options:** * **A. Heparin–Protamine:** This is an example of **Chemical Antagonism**. Protamine (strongly basic) reacts chemically with Heparin (strongly acidic) to form an inactive complex, neutralizing its effect without involving receptors. * **C. Adrenaline–Phenoxybenzamine:** This is **Pharmacological Antagonism**. Phenoxybenzamine is a non-competitive antagonist that binds to the same alpha-receptors that Adrenaline targets. * **D. Physostigmine–Acetylcholine:** This is **Synergism/Potentiation**. Physostigmine inhibits the enzyme acetylcholinesterase, preventing the breakdown of Acetylcholine, thereby increasing its concentration and effect. **3. NEET-PG High-Yield Pearls:** * **Most Common Example:** The most frequently asked example of physiological antagonism is **Adrenaline vs. Histamine** on bronchial smooth muscle (Adrenaline causes bronchodilation via $\beta_2$ receptors; Histamine causes bronchoconstriction via $H_1$ receptors). * **Glucagon vs. Insulin:** Another high-yield example (Glucagon increases blood sugar; Insulin decreases it). * **Key Distinction:** Unlike competitive antagonism, physiological antagonism cannot be completely overcome by increasing the dose of the agonist because the drugs work on entirely different systems.
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