Antimicrobial Agents Practice Questions

Q: Which of the following antimicrobial agents does not require dose reduction in patients with renal failure?

Rifampicin. **Explanation:** The primary factor determining whether a drug requires dose adjustment in renal failure is its **route of elimination**. Drugs primarily excreted by the kidneys require dose reduction to prevent toxicity, whereas drugs metabolized by the liver or excreted via bile do not. **1. Why Rifampicin is correct:** Rifampicin is a highly lipid-soluble drug that undergoes extensive **hepatic metabolism** (deacetylation) and is primarily excreted through the **bile and feces**. Since its clearance is independent of renal function, it is safe to use in standard doses in patients with renal impairment. **2. Why the other options are incorrect:** * **Fluconazole:** Unlike other azoles (like Itraconazole), Fluconazole is primarily excreted unchanged in the urine (>80%). It requires significant dose reduction in renal failure. * **Vancomycin:** This glycopeptide is almost exclusively excreted by glomerular filtration. It is highly nephrotoxic; doses must be adjusted based on creatinine clearance or trough levels. * **Imipenem:** This carbapenem is eliminated renally. Furthermore, it is co-administered with Cilastatin to prevent its breakdown by renal dehydropeptidase-I. Accumulation in renal failure increases the risk of seizures. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Antimicrobials safe in Renal Failure:** "**D**on't **C**hange **M**edicine **L**evels" — **D**oxycycline, **C**eftriaxone/Cefoperazone, **M**acrolides (Erythromycin/Azithromycin), **L**inezolid/Lincosamides (Clindamycin). * **Rifampicin** is a potent **Inducer** of Cytochrome P450 enzymes, leading to numerous drug interactions (e.g., reducing the efficacy of OCPs and Warfarin). * **Doxycycline** is the tetracycline of choice in renal failure because it is excreted via the gut.

Q: After taking certain medications, a patient noticed yellowing of the eyes. Which of the following most often causes cholestatic jaundice?

Erythromycin estolate. **Explanation:** The correct answer is **Erythromycin estolate**. **1. Why Erythromycin estolate is correct:** Cholestatic jaundice is a classic, well-documented hypersensitivity reaction specifically associated with the **estolate salt** of Erythromycin. It typically manifests 10–20 days after starting therapy. The mechanism involves a combination of direct hepatotoxicity and an immune-mediated (allergic) reaction, leading to bile stasis. Patients present with upper abdominal pain, fever, and yellowing of the sclera (icterus), mimicking acute cholecystitis. Interestingly, other salts of erythromycin (like stearate or succinate) rarely cause this condition. **2. Analysis of Incorrect Options:** * **A. Isoniazid (INH):** Causes **hepatocellular necrosis** (elevated ALT/AST) rather than cholestasis. It is metabolized to acetylhydrazine, which causes direct liver injury. * **C. Pyrazinamide:** This is the **most hepatotoxic** first-line anti-tubercular drug. Like INH, it typically causes dose-dependent hepatocellular damage. * **D. Ethionamide:** A second-line anti-tubercular drug that is structurally related to INH; it causes hepatitis in about 5% of patients but is not the classic cause of cholestatic jaundice compared to erythromycin estolate. **3. NEET-PG High-Yield Pearls:** * **Cholestatic Jaundice triad:** Erythromycin estolate, Chlorpromazine, and Methyltestosterone (Anabolic steroids). * **Oral Contraceptives:** Can also cause cholestasis but are usually associated with "painless" jaundice. * **Drug of Choice:** Erythromycin is the drug of choice for atypical pneumonias (Legionella) and in penicillin-allergic patients for syphilis/gonorrhea. * **Prokinetic effect:** Erythromycin acts on **motilin receptors**, making it useful for diabetic gastroparesis.

Q: Combination of sulfamethoxazole and trimethoprim is synergistic due to which of the following mechanisms?

Sequential blockade in folate metabolism. The combination of **Sulfamethoxazole and Trimethoprim** (known as Co-trimoxazole) is a classic example of **pharmacodynamic synergy** through **sequential blockade** of a metabolic pathway. ### Why Option B is Correct Bacteria cannot absorb preformed folic acid and must synthesize it de novo for DNA synthesis. The combination inhibits two consecutive steps in this pathway: 1. **Sulfamethoxazole:** A structural analog of PABA, it competitively inhibits the enzyme **Dihydropteroate Synthase**. 2. **Trimethoprim:** Inhibits the enzyme **Dihydrofolate Reductase (DHFR)**, preventing the reduction of dihydrofolate to tetrahydrofolate (the active form). By attacking two different points in the same biosynthetic chain, the combination becomes **bactericidal**, whereas the individual drugs are usually only bacteriostatic. This also reduces the development of bacterial resistance. ### Why Other Options are Incorrect * **Option A:** This mechanism describes the synergy between **Cell Wall Inhibitors (e.g., Penicillins)** and **Aminoglycosides**, where the cell wall damage allows better entry of the aminoglycoside. * **Option C:** This describes the mechanism of **Beta-lactamase inhibitors** (e.g., Clavulanic acid) when combined with Penicillins. * **Option D:** While inhibiting efflux pumps is a strategy to reverse resistance, it is not the mechanism of action for Co-trimoxazole. ### High-Yield NEET-PG Pearls * **Ratio:** The drugs are administered in a **1:5 ratio** (80mg Trimethoprim : 400mg Sulfamethoxazole) to achieve a **1:20 ratio** in the plasma, which is the optimal synergistic concentration. * **Drug of Choice (DOC):** Co-trimoxazole remains the DOC for *Pneumocystis jirovecii* pneumonia and *Nocardia* infections. * **Adverse Effects:** Watch for **Stevens-Johnson Syndrome (SJS)** due to the sulfonamide component and **megaloblastic anemia** due to folate deficiency.

Q: Which anti-tubercular drug is associated with psychosis?

Isoniazid. **Explanation:** **Isoniazid (INH)** is the correct answer because it interferes with the metabolism of Vitamin B6 (Pyridoxine). INH inhibits the enzyme *pyridoxine phosphokinase*, preventing the conversion of pyridoxine to its active form, pyridoxal phosphate. This deficiency leads to decreased synthesis of neurotransmitters like GABA. Clinically, this manifests as **peripheral neuropathy** (most common) and **neuropsychiatric side effects**, including psychosis, irritability, and seizures. **Analysis of Incorrect Options:** * **Rifampicin:** Primarily associated with hepatotoxicity and the harmless orange-red discoloration of body fluids (urine, sweat, tears). It is a potent inducer of cytochrome P450 enzymes. * **Ethambutol:** Its hallmark side effect is **optic neuritis**, resulting in decreased visual acuity and red-green color blindness. It is generally not associated with CNS or psychiatric symptoms. * **Streptomycin:** An aminoglycoside that causes **ototoxicity** (vestibular and auditory nerve damage) and nephrotoxicity. It does not cross the blood-brain barrier effectively to cause psychosis. **High-Yield Clinical Pearls for NEET-PG:** * **Prophylaxis:** To prevent INH-induced neuropathy and psychosis, **Pyridoxine (10–50 mg/day)** is co-administered, especially in high-risk groups (alcoholics, diabetics, and pregnant women). * **Metabolism:** INH is metabolized by **Acetylation**. "Slow acetylators" are at a higher risk of peripheral neuropathy and psychosis. * **Other Psychotropic TB Drugs:** While not in the options, **Cycloserine** is another potent cause of "TB-induced madness" or psychosis. Ethionamide may also cause psychiatric disturbances.

Q: Which drug is contraindicated in infectious mononucleosis?

Ampicillin. **Explanation:** The correct answer is **Ampicillin (Option A)**. **Why Ampicillin is contraindicated:** Infectious Mononucleosis (IM) is caused by the **Epstein-Barr Virus (EBV)**. When patients with IM are mistakenly treated with aminopenicillins (like **Ampicillin** or **Amoxicillin**) for a suspected bacterial sore throat, a characteristic **maculopapular, pruritic rash** develops in approximately 70–90% of cases. This is not a true IgE-mediated allergy but rather a hypersensitivity reaction resulting from the interaction between the virus and the drug. Therefore, these drugs are strictly avoided in suspected EBV infections. **Analysis of Incorrect Options:** * **B. Doxycycline:** This tetracycline is not associated with a specific hypersensitivity reaction in IM. It is used for atypical pneumonias and rickettsial infections. * **C. Atropine:** An anticholinergic drug used to treat bradycardia or organophosphate poisoning; it has no relevance to the pathophysiology of IM. * **D. Gentamicin:** An aminoglycoside used for Gram-negative infections. While nephrotoxic and ototoxic, it does not trigger the specific rash seen with aminopenicillins in IM. **NEET-PG High-Yield Pearls:** * **The "Ampicillin Rash":** It typically appears 5–10 days after starting the drug. * **Diagnostic Test for IM:** The **Paul-Bunnell Test** (Heterophile antibody test) is the classic gold standard for diagnosis. * **Clinical Triad of IM:** Fever, pharyngitis, and lymphadenopathy (especially posterior cervical). * **Management:** Treatment is primarily supportive. Avoid contact sports for 3–4 weeks due to the risk of **splenic rupture**.

Q: Which drug is active against bacteria producing New Delhi beta-lactamase?

Colistin. **Explanation:** **1. Why Colistin is the Correct Answer:** New Delhi Metallo-beta-lactamase (NDM-1) is a potent enzyme that confers resistance to almost all beta-lactam antibiotics, including carbapenems. It belongs to **Ambler Class B** beta-lactamases, which utilize zinc at their active site to hydrolyze the beta-lactam ring. **Colistin (Polymyxin E)** is a "last-resort" cyclic polypeptide antibiotic that works by a detergent-like mechanism, disrupting the outer cell membrane of Gram-negative bacteria. Since its mechanism of action is independent of the beta-lactam ring or cell wall synthesis, it remains highly effective against NDM-producing multidrug-resistant (MDR) organisms like *E. coli* and *Klebsiella pneumoniae*. **2. Why Other Options are Incorrect:** * **Meropenem (Carbapenem):** NDM-1 is specifically a carbapenemase. It renders carbapenems ineffective, making this a classic "decoy" option. * **Cephalosporins & Penicillins:** These are traditional beta-lactams. NDM-1 producers also typically carry other resistance genes (like ESBLs) that hydrolyze all generations of cephalosporins and penicillins. **3. High-Yield Clinical Pearls for NEET-PG:** * **The "Zinc" Factor:** NDM-1 is inhibited by **EDTA** (a chelating agent) in vitro because it requires zinc ions to function. It is **NOT** inhibited by clavulanic acid or tazobactam. * **Aztreonam Exception:** NDM-1 does not technically hydrolyze Aztreonam (a monobactam). However, most NDM-producing bacteria simultaneously produce other enzymes that *do* destroy Aztreonam, so it is rarely used alone. * **Newer Combinations:** **Ceftazidime-Avibactam** is NOT effective against NDM (Class B). However, **Aztreonam-Avibactam** is a promising combination for these infections. * **Colistin Toxicity:** Always remember the two major side effects: **Nephrotoxicity** (most common) and **Neurotoxicity** (paresthesia, muscle weakness).

Q: What is an indication for stopping rifampicin?

Thrombocytopenia. **Explanation:** Rifampicin is a key bactericidal drug in the RNTCP (National Tuberculosis Elimination Program) regimen. The decision to stop the drug depends on whether the side effect is a minor intolerance or a major toxic reaction. **Why Thrombocytopenia is the correct answer:** Thrombocytopenia is a **Type II Hypersensitivity reaction** caused by Rifampicin-induced antibodies that destroy platelets. It is considered a **major adverse effect**. If a patient develops purpura, petechiae, or a significant drop in platelet count, Rifampicin must be **stopped immediately and never restarted**, as rechallenge can lead to fatal bleeding or acute renal failure. **Analysis of Incorrect Options:** * **Hepatitis (Option A):** While Rifampicin is hepatotoxic, it is usually continued unless the patient becomes clinically jaundiced or serum transaminases (ALT/AST) rise to **>5 times** the upper limit of normal (asymptomatic) or **>3 times** (symptomatic). It is often managed by temporary suspension rather than permanent discontinuation. * **Visual Loss (Option B):** This is the classic side effect of **Ethambutol** (Optic neuritis), not Rifampicin. * **Peripheral Neuropathy (Option C):** This is the classic side effect of **Isoniazid (INH)** due to Vitamin B6 (Pyridoxine) deficiency. **High-Yield Clinical Pearls for NEET-PG:** * **Flu-like syndrome:** Occurs with intermittent (twice weekly) dosing of Rifampicin. * **Orange-colored secretions:** A harmless side effect (urine, sweat, tears); patients should be reassured. * **Enzyme Induction:** Rifampicin is a potent **CYP450 inducer**, reducing the efficacy of OCPs, warfarin, and protease inhibitors. * **Mnemonic for Rifampicin:** **4 R's** — **R**NA polymerase inhibitor, **R**ed-orange secretions, **R**evs up microsomal enzymes (induction), **R**apid resistance if used alone.

Q: All of the following antimicrobial agents are used topically, except?

Griseofulvin. ### Explanation The correct answer is **Griseofulvin**. **1. Why Griseofulvin is the Correct Answer:** Griseofulvin is an antifungal agent that is **exclusively administered orally**. It is ineffective when applied topically because its mechanism of action depends on systemic absorption and subsequent deposition in keratin precursor cells. Once bound to keratin, it makes the skin, hair, and nails resistant to fungal invasion. It is primarily used for dermatophytosis (Tinea infections), particularly **Tinea capitis**, where topical therapy is often insufficient. **2. Analysis of Incorrect Options:** * **Clotrimazole (Option A):** An imidazole antifungal used extensively as a topical cream, powder, or lotion for superficial fungal infections like Tinea corporis, Tinea pedis, and cutaneous candidiasis. * **Nystatin (Option C):** A polyene antifungal that is too toxic for systemic use. It is used **topically** (creams/ointments) or as an oral suspension (not absorbed systemically) for *Candida* infections of the skin and mucous membranes. * **Miconazole (Option D):** Another imidazole derivative used topically for various dermatomycoses and vulvovaginal candidiasis. **3. NEET-PG High-Yield Pearls:** * **Mechanism of Action:** Griseofulvin interferes with **microtubule function**, inhibiting mitosis (metaphase arrest). * **Absorption:** Its absorption is significantly enhanced when taken with a **fatty meal**. * **Contraindications:** It is a known **CYP450 inducer** and is contraindicated in patients with **Porphyria** (it induces ALA synthase) and systemic lupus erythematosus (SLE). * **Drug of Choice:** While terbinafine has largely replaced it, Griseofulvin remains a classic choice for Tinea capitis in children.

Q: Which of the following antimalarial drugs can cause neuropsychiatric reactions and convulsions?

Mefloquine. **Explanation:** **Mefloquine** is the correct answer because it is well-known for its significant **neuropsychiatric side effects**. It is a quinoline-methanol derivative used for both the prophylaxis and treatment of multidrug-resistant *P. falciparum*. The drug has a long half-life (~2–3 weeks) and can cross the blood-brain barrier, leading to adverse effects ranging from dizziness and insomnia to severe reactions like **psychosis, hallucinations, anxiety, depression, and convulsions**. Due to these risks, it is contraindicated in patients with a history of epilepsy or psychiatric disorders. **Analysis of Incorrect Options:** * **Halofantrine:** Its primary clinical concern is **cardiotoxicity**, specifically **QT interval prolongation**, which can lead to fatal arrhythmias (Torsades de pointes). * **Lumefantrine:** Generally well-tolerated and used in combination with Artemether (ACT). While it can cause minor GI upset or headache, it lacks the severe neuropsychiatric profile of Mefloquine. * **Artemisinin derivatives:** These are the fastest-acting antimalarials. Their main side effects are usually mild (nausea, abdominal pain), though rare delayed hemolytic anemia can occur. They do not typically cause convulsions. **High-Yield Clinical Pearls for NEET-PG:** * **Mefloquine Black Box Warning:** The FDA has issued a warning regarding its permanent neurologic and psychiatric side effects. * **Safe in Pregnancy:** Mefloquine is considered safe for use in the **second and third trimesters** (and often the first, if the benefit outweighs the risk) for prophylaxis in travelers. * **Contraindications:** Avoid in patients with **seizures, depression, schizophrenia**, or those in jobs requiring fine motor coordination (e.g., pilots).

Question 1

Which of the following antimicrobial agents does not require dose reduction in patients with renal failure?

Accepted Answer

Rifampicin

Answer

Fluconazole

Answer

Vancomycin

Answer

Imipenem

Question 2

After taking certain medications, a patient noticed yellowing of the eyes. Which of the following most often causes cholestatic jaundice?

Accepted Answer

Erythromycin estolate

Answer

Isoniazid (INH)

Answer

Pyrazinamide

Answer

Ethionamide

Question 3

Combination of sulfamethoxazole and trimethoprim is synergistic due to which of the following mechanisms?

Accepted Answer

Sequential blockade in folate metabolism

Answer

Enhanced entry into bacteria

Answer

Inhibition of drug inactivation enzymes

Answer

Inhibition of drug efflux

Question 4

Which anti-tubercular drug is associated with psychosis?

Accepted Answer

Isoniazid

Answer

Rifampicin

Answer

Ethambutol

Answer

Streptomycin

Question 5

Which drug is contraindicated in infectious mononucleosis?

Accepted Answer

Ampicillin

Answer

Doxycycline

Answer

Atropine

Answer

Gentamicin

Question 6

Which drug is active against bacteria producing New Delhi beta-lactamase?

Accepted Answer

Colistin

Answer

Meropenem

Answer

Cephalosporins

Answer

Penicillins

Question 7

What is an indication for stopping rifampicin?

Accepted Answer

Thrombocytopenia

Answer

Hepatitis

Answer

Visual loss

Answer

Peripheral neuropathy

Question 8

All of the following antimicrobial agents are used topically, except?

Accepted Answer

Griseofulvin

Answer

Clotrimazole

Answer

Nystatin

Answer

Miconazole

Question 9

Which of the following antimalarial drugs can cause neuropsychiatric reactions and convulsions?

Accepted Answer

Mefloquine

Answer

Halofantrine

Answer

Lumefantrine

Answer

Artemisinin derivatives

Question 10

All of the following are false statements regarding tigecycline, except:

Accepted Answer

Tigecycline is not preferred in the treatment of Urinary Tract Infections (UTIs).

Answer

Dose reduction is required in renal failure.

Answer

90% of Pseudomonas strains are sensitive to tigecycline.

Answer

Oral and parenteral bioavailability of tigecycline are the same.

Antimicrobial Agents — MCQs

Antimicrobial Agents — MCQs

On this page

Practice by Chapter

Want unlimited practice?