Antimicrobial Agents — MCQs

Antimicrobial Agents Indian Medical PG Practice Questions and MCQs

Question 771: Quinupristin and Dalfopristin belong to which of the following class of drugs?

A. Macrolides
B. Lincosamides
C. Streptogramins (Correct Answer)
D. Chloramphenicol

Explanation: Quinupristin and Dalfopristin are semi-synthetic derivatives of pristinamycin and belong to the Streptogramin class of antibiotics [1], [2]. They are typically administered together in a fixed-dose combination (30:70 ratio), acting synergistically to inhibit bacterial protein synthesis [1], [2]. * **Mechanism of Action:** They bind to different sites on the 50S ribosomal subunit [1]. Dalfopristin (Streptogramin A) induces a conformational change that enhances the binding of Quinupristin (Streptogramin B). Individually they are bacteriostatic, but together they are bactericidal [2]. **Analysis of Incorrect Options:** * **A. Macrolides (e.g., Erythromycin, Azithromycin):** While they also bind to the 50S subunit, they belong to a distinct chemical class characterized by a macrocyclic lactone ring. * **B. Lincosamides (e.g., Clindamycin):** These bind to the 50S subunit and share a similar binding site with Macrolides and Streptogramins (leading to cross-resistance), but they are chemically distinct [2]. * **D. Chloramphenicol:** This is a broad-spectrum antibiotic that inhibits peptidyl transferase on the 50S subunit but is not related to the streptogramin family. **High-Yield Clinical Pearls for NEET-PG:** * **Spectrum:** Primarily used for Gram-positive cocci, specifically Vancomycin-resistant *Enterococcus faecium* (VRE) and MRSA. Note: They are *not* effective against *Enterococcus faecalis*. * **Adverse Effects:** Significant for arthralgia and myalgia (most common reason for discontinuation) and hyperbilirubinemia. * **Drug Interactions:** They are potent inhibitors of CYP3A4, which can increase levels of drugs like cyclosporine and warfarin. * **Administration:** Must be given via a central venous catheter to avoid severe thrombophlebitis.

Question 772: What is the latest drug used for the treatment of multidrug-resistant tuberculosis (MDR-TB) patients?

A. Bedaquiline (Correct Answer)
B. Amithiozone
C. Capreomycin
D. Linezolid

Explanation: **Explanation:** **Bedaquiline** is the correct answer as it represents a paradigm shift in the management of Multidrug-Resistant Tuberculosis (MDR-TB). It is a diarylquinoline that inhibits the enzyme **mycobacterial ATP synthase**, effectively "starving" the bacteria of energy. It was the first new TB drug class approved by the FDA in over 40 years and is now a core component of the WHO-recommended all-oral shorter regimens for MDR-TB. **Analysis of Options:** * **Amithiozone (Thiacetazone):** An older bacteriostatic drug rarely used today due to severe adverse effects, including Stevens-Johnson Syndrome, especially in HIV-positive patients. * **Capreomycin:** A cyclic peptide antibiotic (injectable) formerly used as a second-line agent. Current WHO guidelines prioritize all-oral regimens, moving injectables like Capreomycin to "Group C" (last resort) or phasing them out entirely. * **Linezolid:** While highly effective and used in MDR-TB regimens (Group A), it is an oxazolidinone originally developed for Gram-positive cocci. Bedaquiline is more "novel" in its specific indication and mechanism for TB. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism of Action:** Inhibits the proton pump of ATP synthase (encoded by the *atpE* gene). * **Black Box Warning:** Bedaquiline can cause **QT interval prolongation**. It should be monitored closely with ECGs, especially if co-administered with Clofazimine or Delamanid. * **Metabolism:** It is metabolized by **CYP3A4**; hence, its levels are decreased by Rifampicin (a potent inducer). * **BPaL Regimen:** A modern, highly effective 6-month regimen for highly resistant TB consisting of **B**edaquiline, **P**retomanid, and **L**inezolid.

Question 773: All of the following are direct-acting antiviral agents for Hepatitis C virus (HCV) infections except?

A. Ledipasvir
B. Beclabuvir
C. Entecavir (Correct Answer)
D. Paritaprevir

Explanation: The core concept tested here is the classification of antiviral drugs based on their target virus. **Entecavir** is a potent nucleoside analog (guanosine analog) used primarily for the treatment of **Chronic Hepatitis B Virus (HBV)** infection [2]. It works by inhibiting HBV DNA polymerase [2]. It has no clinical efficacy against the Hepatitis C Virus (HCV) [1]. **Direct-Acting Antivirals (DAAs) for HCV** are classified based on their molecular targets within the HCV replication cycle [1]: * **NS5A Inhibitors (Suffix "-asvir"):** **Ledipasvir** (Option A) belongs to this class. These drugs inhibit the NS5A replication complex. * **NS5B Polymerase Inhibitors (Suffix "-buvir"):** **Beclabuvir** (Option B) is a non-nucleoside NS5B polymerase inhibitor. Other examples include Sofosbuvir. * **NS3/4A Protease Inhibitors (Suffix "-previr"):** **Paritaprevir** (Option D) belongs to this class. These drugs prevent the cleavage of the viral polyprotein. **High-Yield Clinical Pearls for NEET-PG:** 1. **Mnemonic for HCV DAAs:** * **P**revir = **P**rotease (NS3/4A) * **A**svir = NS5**A** * **B**uvir = NS5**B** (Polymerase) 2. **Entecavir** is a first-line agent for HBV due to its high genetic barrier to resistance, unlike Lamivudine [2]. 3. **Ribavirin** is an older, indirect-acting antiviral used for HCV, but it is not classified as a "Direct-Acting Antiviral" (DAA) [3]. 4. **Sofosbuvir** is the "backbone" of most modern HCV regimens and is a nucleotide NS5B inhibitor.

Question 774: All of the following are nucleoside reverse transcriptase inhibitors (RTIs), EXCEPT?

A. Zidovudine
B. Lamivudine
C. Nevirapine (Correct Answer)
D. Didanoside

Explanation: The correct answer is **Nevirapine** because it belongs to the **Non-Nucleoside Reverse Transcriptase Inhibitor (NNRTI)** class, whereas the other options are Nucleoside Reverse Transcriptase Inhibitors (NRTIs). ### 1. Why Nevirapine is the Correct Answer Antiretroviral drugs are classified based on their mechanism of action. [3] * **NRTIs** (like Zidovudine) are prodrugs that act as competitive inhibitors; they are structural analogs of native nucleosides/nucleotides and require **intracellular phosphorylation** to become active. [1] They cause chain termination. [1] * **NNRTIs** (like Nevirapine, Efavirenz, and Delavirdine) are **non-competitive inhibitors**. They bind directly to a specific "pocket" on the HIV-1 Reverse Transcriptase enzyme, inducing a conformational change. They do **not** require phosphorylation and do **not** compete with native nucleotides. ### 2. Analysis of Incorrect Options (NRTIs) * **A. Zidovudine (AZT):** A thymidine analog and the first drug approved for HIV. [4] It is a classic NRTI. [1] * **B. Lamivudine (3TC):** A cytosine analog NRTI also used in Hepatitis B treatment. [2] * **D. Didanosine (ddI):** An adenosine analog NRTI. [5] ### 3. High-Yield Clinical Pearls for NEET-PG * **Zidovudine Side Effect:** Bone marrow suppression (Anemia/Neutropenia) and Myopathy. [1] * **Nevirapine Side Effect:** Severe hepatotoxicity and potentially fatal skin reactions (Stevens-Johnson Syndrome). It was historically used to prevent mother-to-child transmission (MTCT). * **Stavudine/Didanosine:** Associated with peripheral neuropathy and pancreatitis. [5] * **Abacavir:** Linked to hypersensitivity reactions in patients with the **HLA-B*5701** allele. * **Tenofovir:** The only **Nucleotide** (not nucleoside) RTI; often causes renal toxicity (Fanconi syndrome). [4]

Question 775: All of the following are effective in giardiasis except?

A. Co-trimoxazole (Correct Answer)
B. Furazolidine
C. Metronidazole
D. Tinidazole

Explanation: **Explanation:** The treatment of **Giardiasis** (caused by the protozoan *Giardia lamblia*) primarily relies on 5-nitroimidazole derivatives and certain nitrofuran compounds. **1. Why Co-trimoxazole is the correct answer:** **Co-trimoxazole** (Sulfamethoxazole + Trimethoprim) is a folate antagonist primarily used for bacterial infections and *Pneumocystis jirovecii*. It has **no clinical efficacy** against *Giardia lamblia* [2]. Therefore, it is the "except" option in this list. **2. Why the other options are incorrect (Effective agents):** * **Metronidazole:** Historically the drug of choice for giardiasis. It acts by forming free radicals that damage the protozoal DNA [3]. * **Tinidazole:** Currently considered the **first-line drug** (often preferred over metronidazole) due to its longer half-life, allowing for a **single-dose** regimen with higher efficacy and better tolerability [3]. * **Furazolidone:** A nitrofuran derivative effective against Giardia. It is particularly useful in **pediatric populations** because it is available in liquid suspension and is generally well-tolerated in children. **Clinical Pearls for NEET-PG:** * **Drug of Choice:** Tinidazole (Single dose of 2g) is preferred over Metronidazole (5-7 day course) [3]. * **In Pregnancy:** **Paromomycin** (a non-absorbable aminoglycoside) is the preferred agent if treatment is essential during the first trimester, as it avoids systemic toxicity. * **Nitazoxanide:** Another highly effective, broad-spectrum antiprotozoal used for Giardia, especially in children [1]. * **Mechanism Note:** Most anti-giardial drugs (Nitroimidazoles) require reductive activation by the enzyme **pyruvate:ferredoxin oxidoreductase (PFOR)** within the parasite [3].

Question 776: Which of the following statements regarding Colistin is FALSE?

A. It belongs to a group of antimicrobial agents known as polymyxins which were originally isolated from the spore forming soil organism Paenibacillus polymyxa.
B. Colistin is a mixture of polymyxin E1 and E2, two bactericidal pentacationic lipopeptides.
C. All Gram-positive bacteria are susceptible to Colistin. (Correct Answer)
D. Colistin is generally used to treat infections with multidrug-resistant, extensively drug-resistant, and pandrug-resistant bacteria.

Explanation: ### Explanation **1. Why Option C is the Correct (False) Statement:** Colistin (Polymyxin E) is a **narrow-spectrum** antibiotic specifically active against **Gram-negative** bacteria. Its mechanism of action involves binding to the lipid A component of **Lipopolysaccharides (LPS)** in the outer membrane of Gram-negative organisms, acting like a detergent to disrupt membrane integrity. Since **Gram-positive bacteria lack an outer membrane and LPS**, they are inherently **resistant** to Colistin. Therefore, the statement that they are susceptible is false. **2. Analysis of Other Options:** * **Option A:** This is true. Polymyxins (including Polymyxin B and Colistin/Polymyxin E) are derived from the soil bacterium *Paenibacillus polymyxa* (formerly *Bacillus polymyxa*). * **Option B:** This is true. Colistin is commercially available as a mixture of two cyclic lipopeptides: Colistin A (Polymyxin E1) and Colistin B (Polymyxin E2). They are polycationic, allowing them to interact with the negatively charged LPS. * **Option D:** This is true. Due to significant toxicity, Colistin is considered a "drug of last resort" reserved for severe infections caused by MDR/XDR Gram-negative pathogens like *Pseudomonas aeruginosa*, *Acinetobacter baumannii*, and *Klebsiella pneumoniae*. **3. High-Yield Clinical Pearls for NEET-PG:** * **Spectrum Exceptions:** While most Gram-negatives are susceptible, *Proteus*, *Providencia*, *Serratia*, and *Burkholderia* species are **intrinsically resistant** to Colistin. * **Adverse Effects:** The most important dose-limiting toxicities are **Nephrotoxicity** (Acute Tubular Necrosis) and **Neurotoxicity** (paresthesia, muscle weakness, and neuromuscular blockade). * **Administration:** It is administered as a prodrug, **Colistimethate Sodium (CMS)**, for parenteral use to reduce local toxicity. * **Resistance Mechanism:** Modification of the Lipid A target site (e.g., via the *mcr-1* gene) is the primary mechanism of acquired resistance.

Question 777: What is the drug of choice for the treatment of malaria due to P. vivax in a 25-year-old pregnant female?

A. Chloroquine (Correct Answer)
B. Primaquine
C. Sulfadoxine-pyrimethamine
D. Quinine

Explanation: ### Explanation The treatment of malaria in pregnancy requires careful consideration of both efficacy and fetal safety. For **uncomplicated *P. vivax* malaria**, **Chloroquine** remains the drug of choice across all trimesters of pregnancy. **1. Why Chloroquine is Correct:** Chloroquine is highly effective against the erythrocytic stages of *P. vivax*. It has a well-established safety profile in pregnancy and is not associated with teratogenicity or adverse fetal outcomes. In this 25-year-old pregnant patient, it effectively clears the clinical infection. **2. Why the Other Options are Incorrect:** * **Primaquine:** This is the drug of choice for radical cure (preventing relapse by killing hypnozoites) in non-pregnant adults. However, it is **strictly contraindicated in pregnancy** because it can cross the placenta and cause life-threatening hemolysis in a fetus with unknown G6PD status. * **Sulfadoxine-Pyrimethamine (SP):** While used for Intermittent Preventive Treatment (IPTp) in *P. falciparum* endemic areas, it is not the primary choice for treating acute *P. vivax* malaria. * **Quinine:** This is typically reserved for severe malaria or chloroquine-resistant cases. While safe in pregnancy, it is not the first-line choice for uncomplicated *P. vivax* due to its side effect profile (cinchonism). **3. NEET-PG High-Yield Pearls:** * **Radical Cure Delay:** In pregnant patients with *P. vivax*, radical cure with Primaquine must be **deferred until after delivery** and after checking the G6PD status of both mother and infant (if breastfeeding). * **Suppressive Prophylaxis:** To prevent relapses during pregnancy, chloroquine prophylaxis (weekly) may be continued until delivery. * **Severe Malaria in Pregnancy:** Regardless of the species or trimester, **IV Artesunate** is the drug of choice for severe malaria. * **P. falciparum (Uncomplicated):** ACT (Artemisinin-based Combination Therapy) is now recommended in all trimesters (except SP, which is avoided in the first trimester).

Question 778: Why is quinine unsafe in pregnancy?

A. It causes hemolysis (Correct Answer)
B. It causes hypokalemia
C. It causes hyponatremia
D. It causes smooth muscle contraction

Explanation: **Explanation:** **1. Why Hemolysis is the Correct Answer:** Quinine is associated with a rare but severe idiosyncratic reaction known as **"Blackwater Fever."** This condition involves massive intravascular hemolysis, hemoglobinemia, and hemoglobinuria (resulting in dark-colored urine). In the context of pregnancy, patients with **G6PD deficiency** are at a significantly higher risk of acute hemolytic anemia when exposed to quinine. Since pregnancy already places a physiological strain on the hematological system, quinine-induced hemolysis can lead to severe maternal anemia and secondary renal failure. **2. Analysis of Incorrect Options:** * **B & C (Hypokalemia/Hyponatremia):** Quinine does not have a primary effect on electrolyte balance. However, it is notorious for causing **hyperinsulinemia**, which leads to **hypoglycemia** (a high-yield side effect in pregnancy). * **D (Smooth Muscle Contraction):** While quinine was historically (and incorrectly) thought to be an abortifacient due to potential uterine contractions, modern evidence shows it does not significantly stimulate the pregnant uterus at therapeutic doses. **3. NEET-PG High-Yield Pearls:** * **Cinchonism:** The classic triad of quinine toxicity includes tinnitus, high-frequency hearing loss, and dizziness. * **Cardiotoxicity:** Quinine is dextrorotatory to quinidine; it can cause QT prolongation (Class IA antiarrhythmic effect). * **Drug of Choice:** Despite these risks, IV Artesunate is now the drug of choice for severe malaria in all trimesters. Quinine is reserved as an alternative or used in combination with Clindamycin for uncomplicated malaria in the first trimester if other options are unavailable. * **Hypoglycemia:** Always monitor blood glucose when administering quinine to a pregnant woman.

Question 779: Which fluoroquinolone is associated with fatal hypoglycemia?

A. Lomefloxacin
B. Sparfloxacin
C. Trovafloxacin
D. Gatifloxacin (Correct Answer)

Explanation: **Explanation:** **Gatifloxacin** is the correct answer because it is uniquely associated with severe dysglycemias (both hypoglycemia and hyperglycemia). The underlying mechanism involves the blockade of **ATP-sensitive potassium channels** in pancreatic beta cells. This leads to excessive insulin release (causing fatal hypoglycemia) or, in some cases, inhibition of insulin synthesis (causing hyperglycemia). Due to these life-threatening metabolic side effects, systemic use of Gatifloxacin was withdrawn globally, though it remains available as an ophthalmic preparation. **Analysis of Incorrect Options:** * **Lomefloxacin:** Primarily known for causing significant **phototoxicity** (photosensitivity reactions) and is rarely used due to this profile. * **Sparfloxacin:** Also associated with severe phototoxicity and is notorious for causing **QT interval prolongation**, leading to a risk of Torsades de Pointes. * **Trovafloxacin:** Withdrawn from general use due to severe, unpredictable **hepatotoxicity** (liver failure), though it was once a potent fourth-generation agent. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Fluoroquinolone Toxicity:** * **G**atifloxacin = **G**lucose (Dysglycemia) * **S**parfloxacin = **S**unlight (Phototoxicity) & **S**low heart (QT prolongation) * **T**rovafloxacin = **T**oxic liver (Hepatotoxicity) * **General Side Effects:** Tendon rupture and tendinitis (especially Achilles tendon) are class-wide warnings for fluoroquinolones, particularly in the elderly or those on steroids. * **Contraindication:** They are generally avoided in children and pregnancy due to potential **cartilage damage** (arthropathy).

Question 780: Which of the following is NOT true about aztreonam?

A. Is a beta-lactam antibiotic.
B. Is a monobactam antibiotic.
C. Is active against Pseudomonas.
D. Shows cross-reactivity with other penicillins. (Correct Answer)

Explanation: **Explanation:** **Aztreonam** is a unique member of the beta-lactam family, specifically classified as a **monobactam** (Option A and B are correct). Its structure consists of a standalone beta-lactam ring that is not fused to another ring, unlike penicillins or cephalosporins. The correct answer is **Option D** because aztreonam is famously known for its **lack of cross-reactivity** with other beta-lactams (penicillins, cephalosporins, and carbapenems). This makes it the drug of choice for treating serious infections in patients with a documented **Type-1 hypersensitivity (anaphylaxis) to penicillin**. *Note: The only exception is Ceftazidime, which shares a side chain with aztreonam and may show cross-reactivity.* **Why other options are incorrect:** * **Option A & B:** Aztreonam is indeed a beta-lactam antibiotic belonging to the monobactam subclass. * **Option C:** Its spectrum is highly specific. It is active **only against aerobic Gram-negative bacteria**, including *Pseudomonas aeruginosa*. It has no activity against Gram-positive organisms or anaerobes because it lacks affinity for their specific penicillin-binding proteins (PBPs). **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** Binds to PBP-3 in Gram-negative bacteria, causing cell lysis. * **Spectrum:** "Gram-negative only" (The "Magic Bullet" for Gram-negatives in penicillin-allergic patients). * **Synergy:** Often used with aminoglycosides. * **Resistance:** It is resistant to many beta-lactamases but is inactivated by Extended-Spectrum Beta-Lactamases (ESBLs).

Practice by Chapter

Beta-Lactam Antibiotics

Practice Questions

Aminoglycosides

Practice Questions

Macrolides and Ketolides

Practice Questions

Tetracyclines

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Quinolones

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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

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