Antimicrobial Agents — MCQs

Antimicrobial Agents Indian Medical PG Practice Questions and MCQs

Question 251: Ampicillin is not given in Epstein-Barr virus infection due to what reason?

A. Increased toxicity
B. Skin rash (Correct Answer)
C. Blindness
D. Convulsions

Explanation: **Explanation:** The occurrence of a **maculopapular skin rash** is a classic and highly characteristic reaction when patients with **Infectious Mononucleosis (caused by Epstein-Barr Virus)** are treated with aminopenicillins like **Ampicillin** or Amoxicillin. 1. **Mechanism:** The rash is not a true IgE-mediated Type I hypersensitivity (allergy). Instead, it is a **Type IV delayed hypersensitivity reaction**. It occurs because the EBV infection causes intense proliferation of B-lymphocytes and alters the immune milieu. When Ampicillin is introduced, it acts as a hapten, triggering a T-cell mediated immune response that manifests as a generalized, itchy, erythematous rash approximately 5–10 days after starting the drug. 2. **Incorrect Options:** * **Increased toxicity:** While the rash is an adverse effect, "toxicity" usually refers to organ-specific damage (like hepatotoxicity), which is not the primary concern here. * **Blindness:** Ampicillin has no known association with optic nerve damage or visual loss. * **Convulsions:** While very high doses of Penicillin G can cause seizures (due to GABA antagonism), this is not the specific reason for avoiding Ampicillin in EBV patients. **Clinical Pearls for NEET-PG:** * **Incidence:** The rash occurs in nearly **80–100%** of EBV patients given Ampicillin. * **Diagnostic Value:** If a patient suspected of having "strep throat" develops a diffuse rash after taking Amoxicillin, it strongly suggests the actual diagnosis is **Infectious Mononucleosis**. * **Management:** The rash is self-limiting and does not mean the patient is permanently allergic to Penicillins. * **Other associations:** A similar rash can occur in patients with **Chronic Lymphocytic Leukemia (CLL)** or those taking **Allopurinol** when given Ampicillin.

Question 252: What is the drug of choice for cytomegalovirus infection?

A. Acyclovir
B. Amantadine
C. Ganciclovir (Correct Answer)
D. Idoxuridine

Explanation: **Explanation:** **Ganciclovir** is the drug of choice for the treatment and prophylaxis of **Cytomegalovirus (CMV)** infections, particularly in immunocompromised patients (e.g., HIV/AIDS, transplant recipients). **Mechanism of Action:** Ganciclovir is a nucleoside analogue. It is first phosphorylated to ganciclovir monophosphate by a viral-specific protein kinase (**UL97**) in CMV-infected cells. It is then converted to triphosphate by host cell kinases, which competitively inhibits viral DNA polymerase and terminates viral DNA chain elongation. It is significantly more potent against CMV than acyclovir. **Analysis of Incorrect Options:** * **Acyclovir:** While it is the drug of choice for Herpes Simplex Virus (HSV) and Varicella-Zoster Virus (VZV), it has **poor activity against CMV** because CMV lacks the thymidine kinase enzyme required to activate acyclovir efficiently. * **Amantadine:** This is an anti-influenza agent that acts by inhibiting the M2 ion channel (preventing viral uncoating). It is only effective against **Influenza A** and has no activity against DNA viruses like CMV. * **Idoxuridine:** This is a topical pyrimidine analogue used primarily for **Herpetic keratitis**. Due to its high systemic toxicity, it is not used for systemic infections like CMV. **High-Yield Clinical Pearls for NEET-PG:** * **Valganciclovir:** The oral prodrug of ganciclovir; it is the preferred agent for CMV prophylaxis due to superior bioavailability. * **Side Effects:** The dose-limiting toxicity of Ganciclovir is **bone marrow suppression** (neutropenia and thrombocytopenia). * **Foscarnet/Cidofovir:** Used as second-line agents for CMV when resistance to ganciclovir (UL97 mutation) occurs. Foscarnet does *not* require viral phosphorylation for activation.

Question 253: Which of the following is NOT a disadvantage of INH prophylaxis?

A. Cannot prevent disease in infected individuals (Correct Answer)
B. Not effective
C. Costly
D. Risk of hepatitis

Explanation: ### Explanation **Correct Option: A. Cannot prevent disease in infected individuals** The primary goal of **Isoniazid (INH) prophylaxis** (also known as Latent TB Infection treatment) is specifically to prevent the progression of a latent infection into active clinical disease. By eliminating dormant bacilli in "infected" individuals (those with a positive TST/IGRA but no active symptoms), INH reduces the risk of reactivation by approximately 60–90%. Therefore, stating it *cannot* prevent disease in infected individuals is factually incorrect, making it the "NOT a disadvantage" (i.e., it is actually an advantage). **Analysis of Incorrect Options:** * **B. Not effective:** While INH is highly effective if taken correctly, poor compliance is a major drawback. In public health terms, the "effectiveness" is often lower than "efficacy" due to the long duration (6–9 months) required for prophylaxis. * **C. Costly:** Though the drug itself is cheap, the overall cost of a public health program—including screening, monitoring for side effects, and ensuring long-term adherence—is significant. * **D. Risk of hepatitis:** This is the most serious disadvantage. INH-induced hepatotoxicity is age-related (higher risk in those >35 years) and can be fatal, necessitating regular liver function monitoring. **High-Yield NEET-PG Pearls:** * **Mechanism of Action:** Inhibits **Mycolic acid synthesis** by targeting the *inhA* and *katG* genes. * **Side Effects:** Peripheral neuropathy (prevented by **Pyridoxine/Vitamin B6**), Hepatitis, and Drug-induced Lupus. * **Prophylaxis Dose:** 5 mg/kg (up to 300 mg) daily for 6 to 9 months. * **Drug of Choice:** INH is the DOC for chemoprophylaxis in household contacts of active TB cases and HIV-positive individuals with latent TB.

Question 254: What is the recommended chemoprophylaxis for pregnant females traveling to areas endemic for Plasmodium falciparum?

A. Primaquine
B. Doxycycline
C. Amodiaquine
D. Chloroquine (Correct Answer)

Explanation: **Explanation:** The primary goal of chemoprophylaxis in pregnancy is to provide effective protection against malaria while ensuring fetal safety. **Why Chloroquine is correct:** Chloroquine is considered the safest antimalarial drug during all trimesters of pregnancy. In areas where *Plasmodium falciparum* remains sensitive to it, Chloroquine is the drug of choice for prophylaxis. It does not possess teratogenic potential and has a long-established safety profile for both the mother and the fetus. **Why the other options are incorrect:** * **Primaquine:** It is strictly **contraindicated** in pregnancy. It can cross the placenta and cause life-threatening hemolysis in the fetus if the fetus is G6PD deficient (even if the mother is not). * **Doxycycline:** It is contraindicated in pregnancy (Category D). As a tetracycline, it can cause permanent discoloration of the fetus's teeth and inhibit bone growth. * **Amodiaquine:** While used in some treatment combinations (ACTs), it is not a standard first-line agent for prophylaxis in travelers due to a higher risk of hepatotoxicity and agranulocytosis compared to other agents. **High-Yield Clinical Pearls for NEET-PG:** 1. **Chloroquine-Resistant Areas:** If the traveler is going to a region with Chloroquine-resistant *P. falciparum*, **Mefloquine** is the recommended prophylactic agent for pregnant women (safe in all trimesters). 2. **Atovaquone-Proguanil:** Generally avoided in pregnancy due to lack of sufficient safety data. 3. **Treatment vs. Prophylaxis:** For the *treatment* of uncomplicated falciparum malaria in the 1st trimester, Quinine + Clindamycin is preferred; in the 2nd and 3rd trimesters, ACT (Artesunate + Lumefantrine) is the standard.

Question 255: Fluoroquinolones act on which of the following?

A. DNA histone proteins
B. DNA gyrase (Correct Answer)
C. cAMP
D. mRNA polymerase

Explanation: ### Explanation **Mechanism of Action:** Fluoroquinolones (e.g., Ciprofloxacin, Levofloxacin) are bactericidal antibiotics that inhibit bacterial DNA synthesis. They target two key enzymes: 1. **DNA Gyrase (Topoisomerase II):** This is the primary target in **Gram-negative bacteria**. DNA gyrase is responsible for introducing negative supercoils into the DNA to relieve the torsional stress (positive supercoiling) that occurs ahead of the replicating fork. By inhibiting this enzyme, fluoroquinolones cause double-stranded DNA breaks and cell death. 2. **Topoisomerase IV:** This is the primary target in **Gram-positive bacteria**. It is responsible for the decatenation (separation) of daughter DNA strands after replication. **Analysis of Incorrect Options:** * **A. DNA histone proteins:** Bacteria do not possess histones; their DNA is organized by histone-like proteins (HU proteins). Histones are characteristic of eukaryotic chromatin. * **C. cAMP:** Cyclic AMP is a second messenger involved in intracellular signaling. While some toxins (like Cholera toxin) affect cAMP levels, fluoroquinolones do not. * **D. mRNA polymerase:** This enzyme (specifically DNA-dependent RNA polymerase) is the target of **Rifampin**, not fluoroquinolones. **High-Yield Clinical Pearls for NEET-PG:** * **Resistance:** Occurs primarily via mutations in the *gyrA* or *parC* genes or through efflux pumps. * **Pharmacokinetics:** They exhibit concentration-dependent killing and have excellent tissue penetration (especially in the prostate and lungs). * **Adverse Effects:** * **Tendinitis and Tendon Rupture** (especially Achilles tendon; contraindicated in pregnancy and children due to cartilage toxicity). * **QT interval prolongation** (highest risk with Moxifloxacin). * **Dysglycemia** (Gatifloxacin was withdrawn due to this). * **Drug Interactions:** Absorption is significantly reduced when taken with antacids or iron supplements (chelation).

Question 256: All of the following statements regarding the development of resistance against penicillins is true EXCEPT?

A. Bacteria may develop inactivating enzymes called penicillinases.
B. Bacteria may develop altered penicillin binding proteins.
C. Bacteria may develop alternate metabolic pathways. (Correct Answer)
D. Bacteria may become less permeable to penicillins.

Explanation: **Explanation:** The development of bacterial resistance to penicillins primarily involves mechanisms that interfere with the drug's ability to inhibit cell wall synthesis. **Why Option C is the correct answer:** Penicillins act by inhibiting the cross-linking of peptidoglycan in the bacterial cell wall. Unlike Sulfonamides (which inhibit folic acid synthesis), penicillins do not target metabolic pathways. Therefore, **developing alternate metabolic pathways** is a mechanism of resistance associated with **Sulfonamides and Trimethoprim**, not penicillins. **Why the other options are incorrect:** * **Option A (Inactivating enzymes):** This is the most common mechanism. Bacteria produce **$\beta$-lactamases (penicillinases)** that hydrolyze the $\beta$-lactam ring, rendering the drug inactive (e.g., *Staphylococcus aureus*). * **Option B (Altered PBPs):** Bacteria can modify the target site (Penicillin Binding Proteins). This is the hallmark mechanism for **MRSA** (Methicillin-resistant *S. aureus*) and penicillin-resistant *Streptococcus pneumoniae*. * **Option D (Reduced Permeability):** Gram-negative bacteria can change the size or charge of **porin channels** in their outer membrane, preventing the drug from reaching the PBPs (common in *Pseudomonas*). **High-Yield Clinical Pearls for NEET-PG:** 1. **MRSA Resistance:** Mediated by the **mecA gene**, which encodes an altered PBP called **PBP2a**, which has a low affinity for $\beta$-lactams. 2. **$\beta$-lactamase Inhibitors:** Drugs like Clavulanic acid, Sulbactam, and Tazobactam are used to overcome resistance caused by penicillinases (Option A) but are ineffective against resistance caused by altered PBPs (Option B). 3. **Efflux Pumps:** Another mechanism where bacteria actively pump the drug out of the cell (common in Tetracycline resistance).

Question 257: An elderly patient with tubercular meningitis is not responding to the current treatment. A new combination of drugs needs to be prescribed for a prolonged period. Which one of the following drugs should not be prescribed in this case?

A. Rifampicin
B. Ofloxacin
C. Dehydrostreptomycin (Correct Answer)
D. Pyrazinamide

Explanation: **Explanation:** The correct answer is **Dehydrostreptomycin** because of its high risk of irreversible toxicity, particularly in elderly patients requiring long-term therapy. **1. Why Dehydrostreptomycin is the Correct Answer:** Dehydrostreptomycin is an aminoglycoside derivative that was historically used for tuberculosis. However, it is significantly more **ototoxic** than Streptomycin, specifically causing profound and permanent **cochlear damage (deafness)**. In an elderly patient where renal clearance is often reduced, the risk of accumulation and toxicity is even higher. Furthermore, aminoglycosides generally have poor penetration into the Cerebrospinal Fluid (CSF) unless the meninges are severely inflamed. Due to its severe toxicity profile, it has been largely replaced by safer alternatives. **2. Why the other options are incorrect:** * **Rifampicin:** A bactericidal "first-line" drug and the backbone of TB meningitis treatment. It penetrates the CSF well when meninges are inflamed. * **Ofloxacin:** A fluoroquinolone often used as a "second-line" agent in drug-resistant TB or when first-line drugs are not tolerated. It has excellent CSF penetration. * **Pyrazinamide:** A crucial first-line drug that achieves CSF concentrations nearly equal to plasma concentrations, making it highly effective for tubercular meningitis. **NEET-PG High-Yield Pearls:** * **Drug of Choice for TB Meningitis:** The standard regimen is HRZE (Isoniazid, Rifampicin, Pyrazinamide, Ethambutol). * **CSF Penetration:** Pyrazinamide and Isoniazid have the best CSF penetration among anti-TB drugs. * **Ototoxicity:** Aminoglycosides cause ototoxicity by damaging hair cells; Streptomycin is more vestibulotoxic, while Amikacin and Dehydrostreptomycin are more cochleotoxic. * **Elderly Caution:** Always prioritize drugs with lower toxicity profiles in the elderly due to age-related decline in eighth cranial nerve function and renal reserve.

Question 258: Gynaecomastia is a side effect of which of the following medications?

A. Digitalis
B. Ketoconazole
C. Rifampicin (Correct Answer)
D. Spironolactone

Explanation: **Explanation:** The correct answer is **Rifampicin**. While several drugs are classically associated with gynaecomastia, in the context of this specific question, Rifampicin is the correct choice due to its unique metabolic effects. **1. Why Rifampicin is Correct:** Rifampicin is a potent **inducer of hepatic microsomal enzymes (Cytochrome P450)**. It increases the metabolic clearance of testosterone and other androgens. Additionally, it can alter the ratio of estrogens to androgens in the body. This hormonal imbalance leads to the development of gynaecomastia in some patients undergoing anti-tubercular therapy (ATT). **2. Analysis of Other Options:** * **Digitalis:** While chronic use of Digoxin can cause gynaecomastia (due to its steroid-like structure which can bind to estrogen receptors), it is less frequently tested in this specific context compared to anti-infectives. * **Ketoconazole:** This is a classic cause of gynaecomastia. It inhibits the enzyme **17,20-desmolase**, thereby blocking the synthesis of adrenal and gonadal steroids (testosterone). * **Spironolactone:** This is perhaps the most common cause of drug-induced gynaecomastia. It acts as a **competitive antagonist at androgen receptors** and inhibits testosterone synthesis. *Note: In many standard exams, Spironolactone and Ketoconazole are more common answers. However, if Rifampicin is marked as the key, it highlights the importance of recognizing enzyme induction as a mechanism for hormonal side effects.* **3. High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Gynaecomastia (DISCO):** **D**igoxin, **I**soniazid, **S**pironolactone, **C**imetidine, **O**estrogens/Ketoconazole. * **Rifampicin** is also famous for causing **orange-red discoloration** of body fluids (urine, sweat, tears). * **Ketoconazole** is unique among antifungals for its anti-androgenic side effects, often used off-label for Cushing’s syndrome or prostate cancer.

Question 259: Which antibiotic can cause respiratory failure when used in patients with myasthenia gravis?

A. Telithromycin (Correct Answer)
B. Clindamycin
C. Linezolid
D. Tetracycline

Explanation: **Explanation:** **Telithromycin**, a ketolide antibiotic derived from macrolides, is notorious for causing severe, life-threatening exacerbations of **Myasthenia Gravis (MG)**. The underlying mechanism involves the drug’s ability to block nicotinic acetylcholine receptors at the neuromuscular junction (NMJ). In patients with MG, who already have a reduced number of functional receptors, this blockade can precipitate a "myasthenic crisis," leading to acute respiratory failure. Consequently, the FDA has issued a **Black Box Warning** contraindicating its use in MG patients. **Analysis of Incorrect Options:** * **B. Clindamycin:** While lincosamides can theoretically potentiate neuromuscular blockade, they are not classically associated with the rapid, severe respiratory failure seen with Telithromycin in MG. * **C. Linezolid:** An oxazolidinone primarily known for causing bone marrow suppression (thrombocytopenia) and weak MAO inhibition (risk of Serotonin Syndrome). It does not significantly affect the NMJ. * **D. Tetracycline:** These can occasionally worsen MG by chelating calcium (required for ACh release), but they are far less potent in this regard than ketolides or aminoglycosides. **High-Yield Clinical Pearls for NEET-PG:** * **Aminoglycosides** (e.g., Gentamicin) are the most common class of antibiotics to avoid in MG as they inhibit the pre-synaptic release of Acetylcholine. * **Fluoroquinolones** (e.g., Ciprofloxacin) also carry a black box warning for MG exacerbation. * **Ketolides vs. Macrolides:** Telithromycin is structurally similar to Erythromycin but has a higher affinity for the NMJ receptors, making it significantly more dangerous for MG patients.

Question 260: Which of the following are protease inhibitors?

A. Saquinavir and Nelfinavir (Correct Answer)
B. Nevirapine and Efavirenz
C. Nevirapine and Nelfinavir
D. Saquinavir and Abacavir

Explanation: **Explanation:** Protease Inhibitors (PIs) are a crucial class of Antiretroviral Therapy (ART) that work by inhibiting the viral enzyme **HIV protease**. This enzyme is responsible for cleaving the precursor polyproteins (Gag-Pol) into functional mature proteins. Inhibition results in the production of immature, non-infectious virions. **1. Why Option A is Correct:** Both **Saquinavir** and **Nelfinavir** belong to the Protease Inhibitor class. A high-yield mnemonic for this class is that their names typically end in the suffix **"-navir"** (e.g., Ritonavir, Indinavir, Lopinavir, Atazanavir). **2. Why Other Options are Incorrect:** * **Option B (Nevirapine and Efavirenz):** These are **NNRTIs** (Non-Nucleoside Reverse Transcriptase Inhibitors). They bind directly to the reverse transcriptase enzyme to inhibit viral DNA synthesis. * **Option C (Nevirapine and Nelfinavir):** This is a mixed pair. Nevirapine is an NNRTI, while Nelfinavir is a PI. * **Option D (Saquinavir and Abacavir):** Saquinavir is a PI, but **Abacavir** is an **NRTI** (Nucleoside Reverse Transcriptase Inhibitor), which acts as a chain terminator during DNA synthesis. **High-Yield Clinical Pearls for NEET-PG:** * **Metabolic Side Effects:** PIs are notorious for causing **dyslipidemia, insulin resistance (hyperglycemia), and lipodystrophy** (buffalo hump/central obesity). * **Ritonavir:** Often used in low doses not for its antiviral effect, but as a **"Pharmacokinetic Booster"** because it is a potent inhibitor of CYP3A4, increasing the plasma levels of other PIs. * **Indinavir:** Associated with **nephrolithiasis** (crystalluria); patients must stay well-hydrated. * **Atazanavir:** Known for causing unconjugated hyperbilirubinemia (benign).

Practice by Chapter

Beta-Lactam Antibiotics

Practice Questions

Aminoglycosides

Practice Questions

Macrolides and Ketolides

Practice Questions

Tetracyclines

Practice Questions

Quinolones

Practice Questions

Sulfonamides and Trimethoprim

Practice Questions

Antimycobacterial Drugs

Practice Questions

Antifungal Agents

Practice Questions

Antiviral Drugs

Practice Questions

Antiparasitic Agents

Practice Questions

Principles of Antimicrobial Selection

Practice Questions

Antimicrobial Resistance

Practice Questions

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