Antimicrobial Agents — MCQs

Antimicrobial Agents Indian Medical PG Practice Questions and MCQs

Question 1221: All of the following fluoroquinolones have been withdrawn except?

A. Trovafloxacin
B. Gatifloxacin
C. Gemifloxacin (Correct Answer)
D. Grepafloxacin

Explanation: ### Explanation Fluoroquinolones are a class of broad-spectrum antibiotics that inhibit DNA gyrase and Topoisomerase IV. While many were developed, several have been withdrawn from the market due to severe adverse drug reactions (ADRs). **Why Gemifloxacin is the Correct Answer:** **Gemifloxacin** is a fourth-generation fluoroquinolone primarily used for respiratory tract infections (e.g., CAP and acute exacerbations of chronic bronchitis). Unlike the other options, it remains FDA-approved and available for clinical use. Its most notable side effect is a skin rash (more common in women under 40), but it lacks the life-threatening toxicities that led to the withdrawal of its predecessors. **Analysis of Incorrect Options:** * **Trovafloxacin:** Withdrawn (or severely restricted) due to severe **hepatotoxicity**, leading to acute liver failure. * **Gatifloxacin:** Systemic formulations were withdrawn due to significant **dysglycemias** (both severe hypoglycemia and hyperglycemia), particularly in elderly diabetic patients. (Note: It is still available as ophthalmic drops). * **Grepafloxacin:** Withdrawn globally due to its potential to cause **QT interval prolongation** and associated cardiotoxicity (Torsades de Pointes). **High-Yield Clinical Pearls for NEET-PG:** * **Moxifloxacin:** Known as the "Respiratory Quinolone"; it is metabolized by the liver and does not require dose adjustment in renal failure. * **Ciprofloxacin:** The drug of choice for Anthrax and a potent inhibitor of CYP1A2. * **Common Side Effects:** Tendon rupture (Achilles tendon), phototoxicity, and CNS stimulation (avoid in epilepsy). * **Other withdrawn Quinolones:** Temafloxacin (Hemolytic uremic syndrome) and Sparfloxacin (Phototoxicity and QT prolongation).

Question 1222: A patient presents with high-grade fever and altered sensorium, and is diagnosed with meningococcal meningitis. Which of the following is the most appropriate empirical treatment?

A. Ceftriaxone (Correct Answer)
B. Piperacillin-Tazobactam
C. Penicillin
D. Cotrimoxazole

Explanation: **Explanation:** **Ceftriaxone (Option A)** is the drug of choice for the empirical treatment of meningococcal meningitis. The primary reason is its **excellent cerebrospinal fluid (CSF) penetration**, even when meninges are inflamed, and its high potency against *Neisseria meningitidis*. Unlike Penicillin, Ceftriaxone has a long half-life (allowing once or twice-daily dosing) and covers potential co-pathogens like *Streptococcus pneumoniae* and *Haemophilus influenzae*. **Why other options are incorrect:** * **Penicillin (Option C):** While historically the drug of choice for confirmed sensitive strains, it is no longer used **empirically** due to the global rise of penicillin-resistant *S. pneumoniae* and the requirement for frequent intravenous dosing. * **Piperacillin-Tazobactam (Option B):** This is a broad-spectrum antipseudomonal penicillin. However, it has **poor CNS penetration** and is not indicated for community-acquired meningitis. * **Cotrimoxazole (Option D):** It is not a first-line agent for acute bacterial meningitis due to inferior bactericidal activity compared to third-generation cephalosporins. **High-Yield Clinical Pearls for NEET-PG:** * **Chemoprophylaxis:** For close contacts of a patient with meningococcal meningitis, the drug of choice is **Rifampicin** (alternatives: Ciprofloxacin or Ceftriaxone). * **Steroid Adjunct:** Dexamethasone should be administered **before or with the first dose of antibiotics** to reduce neurological complications (especially in *H. influenzae* and *S. pneumoniae*). * **Waterhouse-Friderichsen Syndrome:** A dreaded complication of meningococcemia involving adrenal hemorrhage and shock.

Question 1223: Which of the following statements about lamivudine is FALSE?

A. Possesses anti-HIV and anti-HBV activity
B. Dose is lower for blocking HIV replication than HBV replication (Correct Answer)
C. Should not be used as monotherapy in HBV/HIV infected patients
D. Anti-HBe seroconversion occurs in a minority of patients

Explanation: **Explanation:** Lamivudine (3TC) is a nucleoside analog that inhibits the reverse transcriptase of HIV and the DNA polymerase of Hepatitis B Virus (HBV). **1. Why Option B is the correct (False) statement:** The statement is factually reversed. **The dose required to inhibit HBV replication (100 mg daily) is significantly lower than the dose required for HIV treatment (300 mg daily).** Using the lower HBV dose in a patient co-infected with HIV can lead to the rapid development of drug-resistant HIV strains (M184V mutation). **2. Analysis of other options:** * **Option A (True):** Lamivudine is unique among older NRTIs for its potent dual activity against both HIV and HBV. * **Option C (True):** Monotherapy should never be used in co-infected patients. If a patient has both HIV and HBV, lamivudine must be part of a full HAART (Highly Active Antiretroviral Therapy) regimen to prevent resistance in both viruses. * **Option D (True):** While lamivudine effectively suppresses HBV DNA levels, HBeAg seroconversion (loss of HBeAg and appearance of anti-HBe) occurs in only about 15–20% of patients after one year of therapy. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** It is a cytosine analog that causes chain termination. * **Resistance:** The most common mutation for lamivudine resistance in both HIV and HBV involves the **M184V/I** mutation in the YMDD motif of the polymerase. * **Safety:** It is one of the least toxic NRTIs; however, abrupt discontinuation in HBV patients can cause a severe "flare-up" of hepatitis. * **Pregnancy:** It is frequently used in pregnancy to prevent vertical transmission of HIV.

Question 1224: Which drug is used for the treatment of kala-azar?

A. Diloxanide furoate
B. Metronidazole
C. Paromomycin (Correct Answer)
D. Spiramycin

Explanation: **Explanation:** **Kala-azar (Visceral Leishmaniasis)** is caused by *Leishmania donovani*. The treatment landscape has evolved due to resistance to traditional drugs like Sodium Stibogluconate. **Why Paromomycin is correct:** **Paromomycin** is an aminoglycoside antibiotic that acts by inhibiting protein synthesis (30S subunit). It has potent leishmanicidal activity and is used as a parenteral treatment for Kala-azar. In India, it is often used in combination with Miltefosine or Liposomal Amphotericin B to increase efficacy and reduce treatment duration. **Analysis of Incorrect Options:** * **Diloxanide furoate:** This is a luminal amoebicide used primarily for asymptomatic cyst passers of *Entamoeba histolytica*. It has no activity against *Leishmania*. * **Metronidazole:** This is the drug of choice for invasive amoebiasis, giardiasis, and trichomoniasis. It is ineffective against the *Leishmania* parasite. * **Spiramycin:** A macrolide antibiotic primarily used for treating Toxoplasmosis, especially in pregnant women to prevent vertical transmission. It is not used for Kala-azar. **NEET-PG High-Yield Pearls:** 1. **Drug of Choice (DOC):** **Liposomal Amphotericin B** (single dose 10mg/kg) is currently the DOC for Kala-azar in the Indian subcontinent. 2. **Miltefosine:** The only **oral** drug available for Kala-azar; however, it is teratogenic (requires strict contraception). 3. **Post-Kala-azar Dermal Leishmaniasis (PKDL):** Usually treated with longer courses of Miltefosine or Amphotericin B. 4. **Sodium Stibogluconate:** Historically the DOC, but now largely abandoned in Bihar/India due to high resistance (up to 60%).

Question 1225: Which of the following antimicrobials is ineffective against Gram-negative bacteria?

A. Cefixime
B. Metronidazole
C. Vancomycin (Correct Answer)
D. Gentamicin

Explanation: **Explanation:** The correct answer is **Vancomycin**. **1. Why Vancomycin is the correct answer:** Vancomycin is a glycopeptide antibiotic that inhibits bacterial cell wall synthesis by binding to the **D-Ala-D-Ala** terminus of nascent peptidoglycan chains. Its lack of activity against Gram-negative bacteria is due to its **large molecular size**. The outer membrane of Gram-negative bacteria acts as a physical barrier; Vancomycin is too bulky to pass through the **porin channels** to reach the peptidoglycan layer. Therefore, its spectrum is strictly limited to Gram-positive organisms (e.g., MRSA, *Enterococcus*). **2. Why the other options are incorrect:** * **Cefixime:** A 3rd-generation oral cephalosporin with extended activity against Gram-negative rods (e.g., *H. influenzae*, *E. coli*). * **Metronidazole:** While primarily known for anaerobes and protozoa, it is highly effective against Gram-negative anaerobes like *Bacteroides fragilis*. * **Gentamicin:** An aminoglycoside that specifically targets the 30S ribosomal subunit. It is highly potent against aerobic Gram-negative bacilli (e.g., *Pseudomonas*, *Klebsiella*). **3. NEET-PG High-Yield Pearls:** * **Red Man Syndrome:** A common side effect of Vancomycin caused by rapid infusion leading to direct histamine release (not a true IgE allergy). * **DOC for MRSA:** Vancomycin remains a primary drug of choice for Methicillin-resistant *Staphylococcus aureus*. * **Oral Vancomycin:** It is not absorbed systemically; therefore, the oral route is used *exclusively* for treating **Pseudomembranous colitis** (*Clostridioides difficile*). * **Mechanism of Resistance:** Alteration of the target site from D-Ala-D-Ala to **D-Ala-D-Lac** (seen in VRSA/VRE).

Question 1226: Which of the following statements about tetracycline is true?

A. Inhibits protein synthesis by binding to the 30s ribosomal subunit in susceptible microorganisms.
B. Tetracyclines possess a chelating property. (Correct Answer)
C. Tetracyclines exhibit anabolic activity.
D. Tetracyclines are infrequently responsible for superinfections.

Explanation: ### Explanation **Correct Option: B. Tetracyclines possess a chelating property.** Tetracyclines are polycyclic compounds that act as strong **chelating agents**. They form stable complexes with divalent and trivalent metal cations such as **Calcium (Ca²⁺), Magnesium (Mg²⁺), Aluminum (Al³⁺), and Iron (Fe²⁺/³⁺)**. This property has two major clinical implications: 1. **Drug Interactions:** Co-administration with antacids, milk, or iron supplements significantly reduces their gastrointestinal absorption. 2. **Tissue Deposition:** They bind to calcium in developing bones and teeth, leading to permanent brown discoloration of teeth and temporary suppression of bone growth. **Why other options are incorrect:** * **Option A:** While tetracyclines do bind to the **30S ribosomal subunit**, the statement is technically incomplete or less "uniquely true" compared to B in the context of competitive exams. However, in many MCQ formats, B is the "most characteristic" chemical property. (Note: Tetracyclines are **bacteriostatic**, inhibiting the binding of aminoacyl-tRNA to the A-site). * **Option C:** Tetracyclines actually exhibit **anti-anabolic** activity. They inhibit protein synthesis in host cells at high doses, leading to an increase in blood urea nitrogen (BUN) and potential azotemia. * **Option D:** Tetracyclines are **broad-spectrum** antibiotics that significantly alter the normal intestinal flora. Consequently, they are **frequently** associated with superinfections, most notably *Candida albicans* (moniliasis) and *Clostridioides difficile* (pseudomembranous colitis). **High-Yield NEET-PG Pearls:** * **Drug of Choice (DOC):** Rickettsial infections, Chlamydia, Cholera, Brucellosis, and Lyme disease. * **Fanconi Syndrome:** Caused by the use of **outdated (expired) tetracyclines**, leading to proximal renal tubular acidosis. * **Contraindications:** Pregnancy (Teratogenic: Category D) and children below 8 years of age. * **Excretion:** Most are excreted renally; however, **Doxycycline** is primarily excreted via feces and is the safest tetracycline in renal failure.

Question 1227: What is the most important element in the treatment of diphtheria?

A. Antitoxin (Correct Answer)
B. Tetracycline
C. Erythromycin
D. Penicillin

Explanation: **Explanation:** The primary pathology of Diphtheria (caused by *Corynebacterium diphtheriae*) is mediated by the **Diphtheria Toxin**, a potent exotoxin that inhibits protein synthesis (via ADP-ribosylation of EF-2), leading to tissue necrosis, pseudomembrane formation, and systemic complications like myocarditis and polyneuritis. **1. Why Antitoxin is the Correct Answer:** The most critical step in management is the **immediate administration of Diphtheria Antitoxin (DAT)**. Antitoxin neutralizes the circulating toxin before it can bind to and enter host cells. Once the toxin is internalized, the antitoxin is no longer effective. Therefore, the prognosis depends on how early the antitoxin is administered, making it the "most important" element for survival. **2. Why Other Options are Incorrect:** * **C. Erythromycin & D. Penicillin:** These are the antibiotics of choice (Erythromycin is preferred). While they stop further toxin production by killing the bacteria and prevent the carrier state, they **do not** neutralize the toxin already present in the system. They are adjuncts, not the primary life-saving treatment. * **B. Tetracycline:** It is not a first-line agent for Diphtheria and lacks the efficacy of macrolides or penicillins in this context. **Clinical Pearls for NEET-PG:** * **Drug of Choice (DOC) for treatment:** Erythromycin (14 days). * **DOC for Prophylaxis (Contacts):** Erythromycin or a single dose of Benzathine Penicillin. * **Schick Test:** Used to determine the immune status of an individual against diphtheria. * **Culture Media:** Löffler's serum slope (rapid growth) and Potassium Tellurite agar (black colonies). * **Important:** Antibiotics are mandatory to prevent transmission, but **Antitoxin is mandatory to prevent death.**

Question 1228: Which of the following drugs used to treat multidrug-resistant tuberculosis (MDR-TB) can cause hypothyroidism?

A. Kanamycin
B. Pyrazinamide
C. Ethionamide (Correct Answer)
D. None of the above

Explanation: Ethionamide is a second-line antitubercular drug (Group C) used in the management of MDR-TB. It is structurally related to Isoniazid and acts by inhibiting mycolic acid synthesis [2].Why Ethionamide causes Hypothyroidism:Ethionamide has a chemical structure similar to thiourea antithyroid drugs (like Methimazole). It inhibits the peroxidase-mediated iodination of tyrosine residues in the thyroid gland, thereby blocking the synthesis of thyroid hormones (T3 and T4). This leads to a compensatory rise in TSH and the development of clinical or subclinical hypothyroidism. This effect is reversible upon discontinuation of the drug and is more common when ethionamide is used in combination with Para-aminosalicylic acid (PAS), which also has antithyroid properties.Analysis of Incorrect Options: * A. Kanamycin: An aminoglycoside used in MDR-TB (Group B). Its primary toxicities are ototoxicity (vestibulocochlear nerve damage) and nephrotoxicity [3]. It does not affect thyroid function. * B. Pyrazinamide: A first-line drug [1]. Its major side effects are hepatotoxicity [4] and hyperuricemia (leading to gouty arthritis) due to inhibition of uric acid excretion [3]. It has no known association with hypothyroidism.High-Yield Clinical Pearls for NEET-PG: * Mnemonic: "Ethionamide and PAS make the thyroid PASS out." (Both cause hypothyroidism). * Other Side Effects of Ethionamide: Severe GI intolerance (most common), metallic taste, and peripheral neuropathy (prevented by Pyridoxine/Vit B6) [1]. * Bedaquiline: A newer MDR-TB drug that causes QT interval prolongation. * Linezolid: Used in XDR-TB; can cause bone marrow suppression (thrombocytopenia) and optic neuritis.

Question 1229: Which of the following is a bacteriostatic anti-tuberculosis drug?

A. Isoniazid (INH)
B. Rifampicin
C. Ethambutol (Correct Answer)
D. Pyrazinamide

Explanation: In the treatment of tuberculosis, drugs are classified based on their mechanism of action as either **bactericidal** (kill the bacteria) or **bacteriostatic** (inhibit growth/multiplication). **Why Ethambutol is correct:** Ethambutol is the only primary anti-tuberculosis drug that is purely **bacteriostatic**. It works by inhibiting the enzyme **arabinosyl transferase**, which interferes with the synthesis of arabinogalactan, a vital component of the mycobacterial cell wall. By disrupting cell wall assembly, it halts the multiplication of *Mycobacterium tuberculosis* but does not directly kill the organism at standard therapeutic doses. **Why the other options are incorrect:** * **Isoniazid (INH):** It is highly **bactericidal** against rapidly dividing bacilli. It inhibits mycolic acid synthesis. * **Rifampicin:** It is a potent **bactericidal** drug that inhibits bacterial DNA-dependent RNA polymerase. It is effective against both rapidly dividing and dormant (persister) bacilli. [1], [2] * **Pyrazinamide:** It is **bactericidal** and uniquely effective in acidic environments (e.g., inside macrophages). It is known for its "sterilizing" activity. [2] **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Bactericidal ATT:** "RIP" (**R**ifampicin, **I**soniazid, **P**yrazinamide, and also Streptomycin). * **Visual Side Effect:** Ethambutol is notorious for causing **optic neuritis**, resulting in decreased visual acuity and **red-green color blindness**. [2] It is generally avoided in young children who cannot undergo visual testing. [2] * **Hyperuricemia:** Both Ethambutol and Pyrazinamide can inhibit uric acid excretion, potentially leading to gouty arthritis. * **Safe in Pregnancy:** Ethambutol is considered safe for use during pregnancy.

Question 1230: Which drug is used in Respiratory Syncytial Virus (RSV) infection in children?

A. Rituximab
B. Palivizumab (Correct Answer)
C. Penicillin
D. None of the above

Explanation: ### Explanation **Correct Option: B. Palivizumab** Palivizumab is a **humanized monoclonal antibody** directed against the **F (fusion) protein** of the Respiratory Syncytial Virus (RSV). By binding to this protein, the drug prevents the virus from fusing with the host cell membrane, thereby inhibiting viral entry and replication. It is primarily used for **immunoprophylaxis** in high-risk pediatric patients (e.g., premature infants, children with bronchopulmonary dysplasia or congenital heart disease) to prevent severe lower respiratory tract infections during RSV season. **Incorrect Options:** * **A. Rituximab:** This is a chimeric monoclonal antibody against **CD20** found on B-cells. It is used in the treatment of Non-Hodgkin Lymphoma, Chronic Lymphocytic Leukemia (CLL), and autoimmune conditions like Rheumatoid Arthritis, but has no antiviral activity. * **C. Penicillin:** This is a beta-lactam antibiotic that inhibits bacterial cell wall synthesis. It is effective against Gram-positive bacteria but is completely ineffective against viruses like RSV. **High-Yield Clinical Pearls for NEET-PG:** * **Ribavirin:** While Palivizumab is used for *prevention*, **Aerosolized Ribavirin** is the drug used for the *treatment* of severe RSV bronchiolitis (though its use is now limited due to toxicity and marginal efficacy). * **Route:** Palivizumab is administered via **intramuscular (IM)** injection once a month during the RSV season. * **Target:** Remember the "F-protein" (Fusion protein) as the specific target for Palivizumab; this is a frequent examiner favorite. * **Nirsevimab:** A newer, long-acting monoclonal antibody recently approved for RSV prevention in all infants entering their first RSV season.

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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