Antimicrobial Agents — MCQs

Antimicrobial Agents Indian Medical PG Practice Questions and MCQs

Question 941: Which of the following statements are true regarding cefepime?

A. It is a fourth-generation cephalosporin (Correct Answer)
B. Once a day dose is sufficient
C. It possesses antipseudomonal action
D. Its dose should not be reduced in renal pathology

Explanation: **Explanation:** **Cefepime** is a parenteral **fourth-generation cephalosporin** [1]. It is characterized by its "zwitterionic" structure, which allows it to penetrate the outer membrane of Gram-negative bacteria rapidly and provides high resistance to many beta-lactamases. * **Why Option A is correct:** Cefepime (along with Cefpirome) belongs to the 4th generation [1]. These agents have an extended spectrum, combining the strong Gram-positive activity of the 1st generation with the potent Gram-negative activity of the 3rd generation. * **Why Option B is incorrect:** Cefepime has a half-life of approximately 2 hours. It requires **twice-daily (q12h)** or thrice-daily (q8h) dosing for serious infections; it is not a once-a-day drug (unlike Ceftriaxone). * **Why Option C is incorrect (Contextual):** While Cefepime **does** possess excellent antipseudomonal action [1], the question asks for the most defining characteristic or the "best" true statement in a single-choice format. In many standard pharmacological classifications, its generation is its primary identifier. *(Note: In some exam patterns, this could be a multiple-correct question, but as a single choice, its classification is the definitive answer).* * **Why Option D is incorrect:** Cefepime is primarily excreted unchanged by the kidneys. Therefore, **dose adjustment is mandatory** in patients with renal impairment to prevent neurotoxicity (e.g., encephalopathy, seizures). **High-Yield Clinical Pearls for NEET-PG:** * **Spectrum:** Highly active against *Pseudomonas aeruginosa* and Enterobacteriaceae producing AmpC beta-lactamases [1]. * **Neurotoxicity:** A unique side effect of Cefepime, especially in elderly or renal-impaired patients, is non-convulsive status epilepticus. * **Comparison:** Unlike 3rd generation drugs, 4th generation cephalosporins are poor inducers of chromosomal beta-lactamases.

Question 942: What is the drug of choice for bacterial vaginosis in a pregnant female?

A. Clindamycin
B. Metronidazole (Correct Answer)
C. Erythromycin
D. Fluconazole

Explanation: **Explanation:** **Bacterial Vaginosis (BV)** is a clinical syndrome resulting from the replacement of normal hydrogen peroxide-producing *Lactobacillus* species in the vagina with high concentrations of anaerobic bacteria (e.g., *Gardnerella vaginalis*, *Prevotella* species, and *Mobiluncus* species). **Why Metronidazole is the Correct Answer:** Metronidazole is the **drug of choice** for Bacterial Vaginosis, regardless of pregnancy status. It is a nitroimidazole that works by inhibiting nucleic acid synthesis in anaerobic bacteria. According to current CDC and WHO guidelines, oral Metronidazole (500 mg twice daily for 7 days) is recommended for pregnant women as it effectively treats the infection and reduces the symptoms of vaginal discharge. Extensive studies have shown **no evidence of teratogenicity** or mutagenicity in humans when used during any trimester of pregnancy. **Analysis of Incorrect Options:** * **A. Clindamycin:** While Clindamycin is an alternative treatment for BV, it is generally reserved for patients allergic to Metronidazole. It is not the primary drug of choice. * **C. Erythromycin:** This macrolide is ineffective against the anaerobic organisms that cause BV. It is often used for *Chlamydia* in pregnancy but not for BV. * **D. Fluconazole:** This is an antifungal agent used to treat **Vulvovaginal Candidiasis** (yeast infections), not bacterial infections. **High-Yield Clinical Pearls for NEET-PG:** * **Diagnosis:** Based on **Amsel’s Criteria** (requires 3 out of 4): 1. Thin, homogenous discharge; 2. Vaginal pH >4.5; 3. Positive Whiff test (fishy odor with KOH); 4. Presence of **Clue cells** on microscopy. * **Pregnancy Complications:** Untreated BV is associated with adverse outcomes like preterm labor, premature rupture of membranes (PROM), and postpartum endometritis. * **Treatment Note:** Unlike Trichomoniasis, **partner treatment is NOT required** for Bacterial Vaginosis as it is not strictly considered an STI.

Question 943: Side effects of isoniazid are all EXCEPT?

A. Hepatitis
B. Optic neuritis
C. Peripheral Neuropathy
D. Thrombocytopenia (Correct Answer)

Explanation: **Explanation:** Isoniazid (INH) is a cornerstone of anti-tubercular therapy (ATT), but it is associated with specific adverse drug reactions (ADRs) that are frequently tested in NEET-PG. **Why Thrombocytopenia is the correct answer:** Thrombocytopenia is **not** a characteristic side effect of Isoniazid. While many drugs can cause bone marrow suppression, thrombocytopenia is classically associated with **Rifampicin** (via immune-mediated mechanisms) or Ethambutol (rarely), rather than INH. **Analysis of incorrect options:** * **Hepatitis (Option A):** This is the most common major toxic effect of INH. It is caused by the metabolite **acetylhydrazine**. The risk increases with age, alcohol consumption, and pre-existing liver disease. * **Optic Neuritis (Option B):** While more classically associated with Ethambutol, INH can also cause optic neuritis (though less frequently). In the context of "all except" questions, it is considered a recognized side effect of INH. * **Peripheral Neuropathy (Option C):** This is a hallmark side effect caused by **pyridoxine (Vitamin B6) deficiency**. INH competes with pyridoxal phosphate and increases its excretion. It is more common in "slow acetylators." **High-Yield Clinical Pearls for NEET-PG:** 1. **Mechanism of Action:** Inhibits mycolic acid synthesis by targeting the *inhA* gene. 2. **Metabolism:** Metabolized by **Acetylation** (Phase II reaction). Slow acetylators are prone to neuropathy; fast acetylators may be prone to hepatotoxicity. 3. **Prophylaxis:** Peripheral neuropathy can be prevented by co-administering **Pyridoxine (10–25 mg/day)**. 4. **Sideroblastic Anemia:** INH can also cause microcytic hypochromic anemia due to interference with heme synthesis (B6 deficiency). 5. **Drug-Induced Lupus:** INH is a well-known cause of Systemic Lupus Erythematosus (SLE), usually associated with anti-histone antibodies.

Question 944: Which of the following agents is a fusion inhibitor?

A. Zidovudine
B. Enfuvirtide (Correct Answer)
C. Nevirapine
D. Ritonavir

Explanation: **Explanation:** **Correct Answer: B. Enfuvirtide** Enfuvirtide is the prototype **fusion inhibitor**. Its mechanism of action involves binding to the **gp41** subunit of the viral envelope glycoprotein. This prevents the conformational change required for the fusion of the HIV viral envelope with the host cell (CD4+ T-cell) membrane, thereby blocking viral entry. It is administered subcutaneously and is typically reserved for treatment-experienced patients with multidrug-resistant HIV. **Analysis of Incorrect Options:** * **A. Zidovudine (AZT):** This is a **Nucleoside Reverse Transcriptase Inhibitor (NRTI)**. It acts as a thymidine analogue that causes premature chain termination during the synthesis of viral DNA by reverse transcriptase. * **C. Nevirapine:** This is a **Non-Nucleoside Reverse Transcriptase Inhibitor (NNRTI)**. It binds directly to the reverse transcriptase enzyme at a site distinct from the active site (allosteric site), inducing a conformational change that inactivates the enzyme. * **D. Ritonavir:** This is a **Protease Inhibitor (PI)**. It inhibits the viral protease enzyme (HIV-1 protease), preventing the cleavage of gag-pol polyproteins into mature, functional proteins, resulting in the production of immature, non-infectious virions. **High-Yield Clinical Pearls for NEET-PG:** * **Maraviroc** is another entry inhibitor, but it is a **CCR5 antagonist** (blocks the host cell co-receptor), whereas Enfuvirtide blocks the viral protein gp41. * **Ritonavir** is rarely used for its own antiviral effect today; it is primarily used as a **"pharmacokinetic booster"** because it is a potent inhibitor of CYP3A4, increasing the plasma levels of other protease inhibitors (e.g., Lopinavir). * **Zidovudine** is the drug of choice for preventing vertical transmission (mother-to-child) of HIV during pregnancy and labor.

Question 945: Which of the following antimicrobial agents is not given in pregnancy?

A. Penicillin G
B. Quinolone (Correct Answer)
C. Cephalosporin
D. Erythromycin

Explanation: **Explanation:** The correct answer is **Quinolones (e.g., Ciprofloxacin, Levofloxacin)**. **1. Why Quinolones are contraindicated:** Quinolones are generally avoided during pregnancy because they have a high affinity for bone and cartilage. Experimental animal studies have shown that these agents can cause **arthropathy** and permanent damage to the **weight-bearing joints** (cartilage erosion) in developing fetuses. They are classified as FDA Pregnancy Category C. **2. Analysis of Incorrect Options:** * **Penicillin G (Option A):** Penicillins are considered the safest antibiotics during pregnancy (Category B). They do not have teratogenic effects and are the drug of choice for conditions like syphilis in pregnant women. * **Cephalosporins (Option C):** Like penicillins, cephalosporins (e.g., Ceftriaxone, Cephalexin) are Category B drugs. They are widely used and considered safe for treating urinary tract infections and respiratory infections in pregnancy. * **Erythromycin (Option D):** Erythromycin (base/stearate) is considered safe (Category B). However, the **Estolate salt** of erythromycin is contraindicated as it may cause cholestatic hepatitis in the mother. **3. NEET-PG High-Yield Pearls (Mnemonic: SAFE / FAST):** * **SAFE Antibiotics in Pregnancy:** **S**ulfonamides (only in mid-trimester), **A**mpicillin/Penicillins, **F**irst-line Cephalosporins, **E**rythromycin. * **Antibiotics to AVOID (Mnemonic: MCAT):** * **M**etronidazole (avoid in 1st trimester). * **C**hloramphenicol (Gray Baby Syndrome). * **A**minoglycosides (Ototoxicity/CN VIII damage). * **T**etracyclines (Discolored teeth and bone growth inhibition). * **Nitrofurantoin:** Safe until 36 weeks; avoid at term due to risk of neonatal hemolysis (G6PD deficiency).

Question 946: Which of the following statements about Acyclovir is false?

A. Useful and efficient only for recurrent herpes infections
B. Inhibits thymidine kinase
C. Doesn't work on other viruses (Correct Answer)
D. Metabolized and excreted through kidneys

Explanation: ### Explanation **Correct Option: C (Doesn't work on other viruses)** This statement is **false** (making it the correct answer for this question). While Acyclovir is most potent against Herpes Simplex Virus (HSV-1 and HSV-2) and Varicella-Zoster Virus (VZV), it also possesses activity against other members of the Herpesviridae family, such as **Epstein-Barr Virus (EBV)** and **Cytomegalovirus (CMV)**, though its clinical efficacy against CMV is limited compared to Ganciclovir. **Analysis of Other Options:** * **Option A (Useful for recurrent infections):** This is a true statement. Acyclovir is highly effective in reducing the duration and severity of both primary and recurrent genital and oral herpes. * **Option B (Inhibits thymidine kinase):** This is a true statement regarding its mechanism. Acyclovir is a prodrug that must be phosphorylated to Acyclovir-monophosphate by **viral Thymidine Kinase** [1], [2]. It then competes with deoxyguanosine triphosphate to inhibit **viral DNA polymerase**, leading to DNA chain termination [1]. * **Option D (Metabolized and excreted through kidneys):** This is true. Acyclovir is primarily excreted unchanged via glomerular filtration and tubular secretion [1], [3]. Dosage adjustment is mandatory in patients with renal impairment to prevent neurotoxicity and crystalline nephropathy. **High-Yield NEET-PG Pearls:** * **Mechanism of Resistance:** Most common cause is the absence or mutation of viral **Thymidine Kinase** [2]. * **Drug of Choice:** For HSV encephalitis, Neonatal HSV, and Herpes Zoster (Shingles). * **Adverse Effect:** Reversible **crystalline nephropathy** (prevented by adequate hydration) and neurotoxicity (tremors, seizures). * **Valacyclovir:** A prodrug of acyclovir with much higher oral bioavailability (the "L-valyl ester" of acyclovir) [3].

Question 947: In cotrimoxazole, what is the ratio of sulphamethoxazole to trimethoprim?

A. 2:1
B. 1:1
C. 5:1 (Correct Answer)
D. 1:5

Explanation: ### Explanation **Correct Answer: C. 5:1** **1. Underlying Medical Concept:** Cotrimoxazole is a fixed-dose combination of **Sulphamethoxazole (SMZ)** and **Trimethoprim (TMP)**. These two drugs act synergistically by blocking sequential steps in bacterial folic acid synthesis (Sequential Blockade). * **The Ratio in the Tablet:** The drugs are combined in a **5:1 ratio** (e.g., 400 mg SMZ + 80 mg TMP). * **The Ratio in the Plasma:** Because Trimethoprim has a significantly larger volume of distribution and better lipid solubility than Sulphamethoxazole, it distributes more widely into tissues. To achieve the optimal synergistic therapeutic concentration in the **plasma (which is 20:1)**, they must be administered in a **5:1 dose ratio**. **2. Analysis of Incorrect Options:** * **Option A (2:1) & B (1:1):** These ratios do not provide the necessary pharmacokinetic balance required to reach the 20:1 steady-state plasma concentration needed for maximal antibacterial activity. * **Option D (1:5):** This reverses the components. Sulphamethoxazole must be the higher dose component because it has a smaller volume of distribution and lower potency per milligram compared to Trimethoprim. **3. NEET-PG High-Yield Clinical Pearls:** * **Mechanism:** SMZ inhibits *Dihydropteroate synthase*; TMP inhibits *Dihydrofolate reductase*. * **Spectrum:** It is the drug of choice for *Pneumocystis jirovecii* pneumonia, *Nocardia*, and *Stenotrophomonas maltophilia*. * **Adverse Effects:** Watch for **megaloblastic anemia** (due to folate deficiency) and **Stevens-Johnson Syndrome** (due to the sulfonamide component). * **Resistance:** Bacteria usually develop resistance by acquiring plasmids that code for altered dihydrofolate reductase enzymes.

Question 948: All of the following are true about the therapy for tuberculosis, except?

A. A 'flu-like syndrome' is usually seen in people taking rifampicin on a daily basis. (Correct Answer)
B. Ethambutol accumulates in renal failure.
C. Hyperuricemia is a recognized side effect of pyrazinamide.
D. Red-green color impairment is an early sign of ethambutol-induced optic neuritis.

Explanation: **Explanation** The correct answer is **A**. The "flu-like syndrome" (fever, chills, malaise, and bone pain) associated with Rifampicin is an **immunological reaction** that occurs primarily when the drug is taken **intermittently** (less than twice weekly) or when therapy is resumed after a long break. It is rarely seen with the standard **daily dosing** used in modern DOTS regimens. **Analysis of Other Options:** * **Option B (Ethambutol in renal failure):** Ethambutol is primarily excreted by the kidneys (approx. 80%). Therefore, it accumulates in patients with renal impairment, necessitating dose adjustment or avoidance to prevent toxicity. * **Option C (Pyrazinamide and Hyperuricemia):** Pyrazinamide (and its metabolite pyrazinoic acid) inhibits the renal excretion of uric acid. This leads to hyperuricemia in most patients, which may occasionally precipitate acute gouty arthritis. * **Option D (Ethambutol and Optic Neuritis):** Retrobulbar neuritis is the most significant side effect of Ethambutol. The earliest clinical manifestation is the loss of **red-green color discrimination**, followed by decreased visual acuity and central scotomas. **High-Yield Clinical Pearls for NEET-PG:** 1. **Rifampicin:** Potent inducer of Cytochrome P450 enzymes; causes harmless orange-red discoloration of urine, sweat, and tears. 2. **Pyrazinamide:** The most hepatotoxic drug among the first-line ATT; it is most active in an acidic medium (intracellularly). 3. **Ethambutol:** The only **bacteriostatic** first-line ATT; all others are bactericidal. It is contraindicated in children too young to undergo visual acuity testing. 4. **Isoniazid (INH):** Causes peripheral neuropathy (prevented by Pyridoxine/Vit B6) and is metabolized via **acetylation** (genetic polymorphism: fast vs. slow acetylators).

Question 949: Gray baby syndrome is caused by which of the following drugs?

A. Penicillin
B. Chloramphenicol (Correct Answer)
C. Rifampicin
D. Erythromycin

Explanation: **Explanation:** **Gray Baby Syndrome** is a serious and potentially fatal adverse reaction associated with the administration of **Chloramphenicol** in neonates, particularly premature infants [2]. **Mechanism of Action:** The syndrome occurs due to the neonate's physiological immaturity in two key areas: 1. **Glucuronosyltransferase Deficiency:** Neonates lack sufficient levels of this hepatic enzyme, which is required to metabolize chloramphenicol via glucuronidation [1]. 2. **Inadequate Renal Excretion:** Immature kidneys cannot effectively excrete the unconjugated drug [2]. This leads to the accumulation of toxic levels of chloramphenicol, which inhibits mitochondrial protein synthesis, resulting in myocardial depression, vascular collapse, and the characteristic "ashen gray" cyanosis. **Analysis of Incorrect Options:** * **A. Penicillin:** Primarily associated with hypersensitivity reactions (Type I IgE-mediated) and seizures at very high doses. * **C. Rifampicin:** Known for causing orange-colored secretions (urine, sweat, tears) and hepatotoxicity. * **D. Erythromycin:** Commonly associated with GI upset and, in neonates, an increased risk of Infantile Hypertrophic Pyloric Stenosis (IHPS). **NEET-PG High-Yield Pearls:** * **Clinical Presentation:** Abdominal distension, vomiting, progressive pallid cyanosis (gray color), and circulatory collapse. * **Other Chloramphenicol Side Effects:** Dose-dependent bone marrow suppression (reversible) and **Aplastic Anemia** (idiosyncratic, irreversible, and most dreaded). * **Drug of Choice:** Chloramphenicol remains a backup for enteric fever (Typhoid) [1] and bacterial meningitis in patients with severe penicillin allergy [2].

Question 950: Select the antibiotic most frequently associated with Gray baby syndrome?

A. Tetracycline
B. Streptomycin
C. Nitrofurantoin
D. Chloramphenicol (Correct Answer)

Explanation: **Explanation:** **Chloramphenicol** is the correct answer because it is classically associated with **Gray Baby Syndrome**, a serious and potentially fatal adverse reaction in neonates (especially premature infants). **Pathophysiology:** The syndrome occurs due to the neonate’s physiological immaturity in two key areas: 1. **Deficiency of Glucuronyl Transferase:** This enzyme is required to conjugate chloramphenicol into its inactive form. 2. **Inadequate Renal Excretion:** Immature kidneys cannot effectively excrete the unconjugated drug. This leads to toxic accumulation of the drug, which interferes with mitochondrial ribosomes, causing myocardial depression and circulatory collapse. Clinical features include abdominal distension, vomiting, progressive pallid cyanosis (giving the "gray" appearance), and flaccidity. **Analysis of Incorrect Options:** * **A. Tetracycline:** Primarily associated with **teratogenicity** (permanent discoloration of teeth and inhibition of bone growth) if used during pregnancy or in children under 8 years. * **B. Streptomycin:** An aminoglycoside known for **ototoxicity** (vestibular/cochlear damage) and **nephrotoxicity**. In neonates, it may cause fetal eighth cranial nerve damage if used during pregnancy. * **C. Nitrofurantoin:** Most commonly associated with **hemolytic anemia** in patients with G6PD deficiency and pulmonary fibrosis with chronic use. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism of Action:** Chloramphenicol inhibits protein synthesis by binding to the **50S ribosomal subunit**. * **Other Side Effects:** It is notorious for causing **Bone Marrow Suppression** (dose-dependent anemia and dose-independent irreversible **Aplastic Anemia**). * **Drug of Choice:** Though limited by toxicity, it remains a drug of choice for **H. influenzae meningitis** (in penicillin-allergic patients) and occasionally for **Typhoid fever** or **Rickettsial infections** where alternatives are unavailable.

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

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Principles of Antimicrobial Selection

Practice Questions

Antimicrobial Resistance

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