Antimicrobial Agents Practice Questions

Q: Which ATT drug causes contact lens staining?

Rifampicin. **Explanation** **Rifampicin** is the correct answer because it is a potent inducer of hepatic enzymes and is excreted through various body fluids, including urine, sweat, saliva, and **tears**. Due to its intrinsic chemical structure, it imparts a characteristic **orange-red discoloration** to these secretions. In patients wearing soft contact lenses, the drug can permanently stain the lenses, leading to patient anxiety and the need for lens replacement. Patients should be counseled to switch to spectacles during the intensive phase of Antitubercular Therapy (ATT). **Analysis of Incorrect Options:** * **Isoniazid (INH):** Primarily associated with peripheral neuropathy (due to Vitamin B6 deficiency) and hepatotoxicity. It does not cause secretion discoloration. * **Streptomycin:** An aminoglycoside known for ototoxicity (vestibulocochlear nerve damage) and nephrotoxicity. It is administered parenterally and does not affect tear color. * **Pyrazinamide:** Most commonly associated with hyperuricemia (leading to gouty arthritis) and hepatotoxicity. It has no effect on contact lenses. **Clinical Pearls for NEET-PG:** * **Ethambutol (The "E" for Eye):** While Rifampicin stains lenses, Ethambutol causes **Retrobulbar Neuritis**, leading to decreased visual acuity and red-green color blindness. * **Rifampicin Mechanism:** Inhibits DNA-dependent RNA polymerase. * **Mnemonic:** Rifampicin makes everything **R**ed/**R**usty (Urine, Tears, Sweat). * **Drug Interactions:** Rifampicin is a powerful **CYP450 inducer**, which can decrease the efficacy of oral contraceptives and warfarin.

Q: Once weekly administration of which of the following antibiotics has prophylactic activity against bacteraemia caused by M.avium complex in AIDS patients?

Azithromycin. **Explanation:**The primary goal of prophylaxis against **Mycobacterium avium complex (MAC)** in HIV/AIDS patients (typically when CD4 counts fall below 50 cells/mm³) is to prevent disseminated infection. **1. Why Azithromycin is correct:**Azithromycin is a macrolide with a exceptionally long tissue half-life (approx. 68 hours). This pharmacokinetic property allows for once-weekly dosing (1200 mg) [1], which significantly improves patient compliance. It is the preferred agent for primary prophylaxis of MAC because it is effective, well-tolerated, and requires infrequent administration. **2. Why the other options are incorrect:** * **Clarithromycin:** While also a first-line agent for MAC, it has a much shorter half-life than azithromycin. It requires **twice-daily (BID)** administration for prophylaxis, making it less convenient than weekly azithromycin. * **Isoniazid:** This is the mainstay for *Mycobacterium tuberculosis* (TB) prophylaxis (Latent TB Infection), but it has no clinical activity against MAC. * **Rifabutin:** This is considered an alternative for MAC prophylaxis if macrolides are not tolerated. However, it must be administered **daily**, carries a risk of uveitis, and involves significant drug-drug interactions via CYP450 induction. **Clinical Pearls for NEET-PG:** * **Initiation Criteria:** MAC prophylaxis is indicated in HIV patients with **CD4 100 cells/mm³ for at least 3 months** in response to ART.

Q: Which of the following drug toxicities can be seen in Crigler-Najjar syndrome?

Atazanavir. **Explanation:** The correct answer is **Atazanavir**. **1. Why Atazanavir is correct:** Atazanavir is a Protease Inhibitor (PI) used in HIV treatment. Its primary metabolic pathway involves inhibition of the enzyme **UGT1A1 (UDP-glucuronosyltransferase)**. This enzyme is responsible for the conjugation of bilirubin in the liver. * **Crigler-Najjar Syndrome** is a genetic disorder characterized by a deficiency (Type II) or total absence (Type I) of UGT1A1. * When a patient with Crigler-Najjar syndrome (or the milder Gilbert syndrome) takes Atazanavir, the drug further inhibits the already deficient enzyme, leading to a significant rise in unconjugated bilirubin and clinical **jaundice/hyperbilirubinemia**. **2. Why the other options are incorrect:** * **Tenofovir (A):** A Nucleotide Reverse Transcriptase Inhibitor (NRTI) primarily associated with **nephrotoxicity** (Fanconi syndrome) and bone mineral density loss, not UGT inhibition. * **Zidovudine (B):** An NRTI known for **bone marrow suppression** (anemia, neutropenia) and mitochondrial toxicity (lactic acidosis). * **Nevirapine (D):** A Non-Nucleoside Reverse Transcriptase Inhibitor (NNRTI) associated with severe **hepatotoxicity** and skin rashes (Stevens-Johnson Syndrome), but it does not specifically target the UGT1A1 pathway like Atazanavir. **Clinical Pearls for NEET-PG:** * **Atazanavir** is often called the "bananavir" in medical mnemonics because it causes yellowing of the skin (jaundice). * **Gilbert Syndrome:** Similar to Crigler-Najjar but milder; Atazanavir is also contraindicated or used with extreme caution here. * **Indinavir:** Another Protease Inhibitor that can cause hyperbilirubinemia and nephrolithiasis (kidney stones).

Q: Which of the following anti-TB drugs is NOT bactericidal?

Ethambutol. ### Explanation In the treatment of Tuberculosis, drugs are classified based on their mechanism of action as either **bactericidal** (killing the bacteria) or **bacteriostatic** (inhibiting growth). **Why Ethambutol is the correct answer:** Ethambutol is the only **bacteriostatic** drug among the first-line anti-TB agents. It works by inhibiting the enzyme **arabinosyl transferase**, which interferes with the synthesis of arabinogalactan, an essential component of the mycobacterial cell wall. Since it only halts the replication process rather than directly killing the bacilli, it is classified as bacteriostatic. **Why the other options are incorrect:** * **Rifampicin:** A potent **bactericidal** drug. It inhibits DNA-dependent RNA polymerase, preventing transcription. It is effective against both rapidly dividing and dormant (persister) bacilli. * **Isoniazid (INH):** Primarily **bactericidal** against rapidly dividing mycobacteria. It inhibits the synthesis of mycolic acids, which are vital for cell wall integrity. * **Pyrazinamide:** A **bactericidal** drug that is particularly effective in acidic environments (e.g., inside macrophages). It is crucial for killing intracellular organisms. **NEET-PG High-Yield Pearls:** * **Mnemonic for First-line Drugs:** **PRIEST** (Pyrazinamide, Rifampicin, Isoniazid, Ethambutol, STreptomycin). All are bactericidal **except** Ethambutol. * **Visual Side Effect:** Ethambutol is notorious for causing **Retrobulbar Neuritis**, leading to decreased visual acuity and **red-green color blindness**. (Remember: **E**thambutol for **E**ye). * **Sterilizing Activity:** Rifampicin and Pyrazinamide have the highest sterilizing activity, which helps in preventing relapses. * **Safe in Renal Failure:** Rifampicin and Isoniazid are primarily metabolized by the liver, while Ethambutol requires dose adjustment in renal impairment.

Q: What is the drug of choice for MRSA skin infection?

Vancomycin. **Explanation:** **Methicillin-resistant Staphylococcus aureus (MRSA)** is a major cause of skin and soft tissue infections (SSTIs). It is characterized by the presence of the **mecA gene**, which alters penicillin-binding proteins (PBP2a), rendering all beta-lactams (except 5th generation cephalosporins) ineffective. **Why Vancomycin is the Correct Answer:** **Vancomycin** (a glycopeptide) remains the traditional **Drug of Choice (DOC)** for serious systemic MRSA infections and complicated SSTIs. It works by inhibiting bacterial cell wall synthesis at the D-Ala-D-Ala terminus. In the context of NEET-PG, unless "oral" or "minor infection" is specified, Vancomycin is the gold standard for MRSA. **Analysis of Incorrect Options:** * **Linezolid (Option D):** While highly effective against MRSA and often used for skin infections due to its excellent tissue penetration, it is generally reserved as a second-line agent or for cases where IV access is difficult (due to 100% oral bioavailability). * **Quinupristin/Dalfopristin (Option C):** These are **Streptogramins (Option A)**. While they cover MRSA, they are primarily reserved for Vancomycin-resistant strains (VRSA) or Vancomycin-resistant Enterococcus (VRE) due to their significant side effect profile (e.g., infusion-site reactions, arthralgia). **High-Yield Clinical Pearls for NEET-PG:** * **Oral DOC for MRSA (Minor infections):** Cotrimoxazole, Clindamycin, or Doxycycline. * **Newer agents for MRSA:** Daptomycin (not used in pneumonia), Telavancin, and Ceftaroline (5th gen Cephalosporin). * **Mupirocin:** Topical drug of choice for MRSA colonization (nasal) and Impetigo. * **Red Man Syndrome:** A common side effect of Vancomycin due to histamine release (managed by slowing the infusion rate).

Q: Which of the following drugs can be used for the treatment of chloroquine-resistant malaria in children?

Clindamycin. **Explanation:** The treatment of chloroquine-resistant *Plasmodium falciparum* requires combination therapy. According to the National Vector Borne Disease Control Programme (NVBDCP) and WHO guidelines, when using Quinine for resistant malaria, it must be paired with a second antibiotic to ensure complete parasite clearance. **Why Clindamycin is Correct:** In children (especially those under 8 years of age) and pregnant women, **Clindamycin** is the drug of choice to be used in combination with Quinine. It acts by inhibiting protein synthesis via the 50S ribosomal subunit and is safe for use in pediatric populations where other options are contraindicated. **Analysis of Incorrect Options:** * **Chloroquine:** By definition, it is ineffective against chloroquine-resistant strains due to the *pfcrt* gene mutation which increases drug efflux from the parasite's food vacuole. * **Doxycycline & Tetracycline:** While these are highly effective against resistant malaria, they are **contraindicated in children under 8 years of age**. These drugs cause permanent dental discoloration (yellowish-brown staining) and can inhibit bone growth by chelating calcium in developing teeth and bones. **NEET-PG High-Yield Pearls:** * **Drug of Choice (DOC):** For uncomplicated chloroquine-resistant malaria in India, the standard treatment is **ACT (Artemisinin-based Combination Therapy)**. * **Quinine + Clindamycin:** This is the specific regimen for resistant malaria in **pregnancy (1st trimester)** and **young children**. * **Mechanism of Resistance:** Chloroquine resistance is primarily due to mutations in the *Plasmodium falciparum* chloroquine resistance transporter (**pfcrt**) gene. * **Side Effect Note:** Always remember that Clindamycin is classically associated with *Pseudomembranous colitis* caused by *C. difficile*.

Q: Streptomycin is useful in the treatment of which of the following conditions?

Granuloma venereum. **Explanation:** Streptomycin, an aminoglycoside, is considered a second-line or alternative treatment for **Granuloma venereum** (also known as Donovanosis), caused by *Klebsiella granulomatis*. While macrolides (Azithromycin) are currently the first-line treatment, streptomycin remains a classic pharmacological association for this condition in competitive exams. **Analysis of Options:** * **Granuloma venereum (Correct):** Characterized by painless, beefy-red ulcerative lesions. Diagnosis is confirmed by identifying **Donovan bodies** (safety-pin appearance) in tissue smears. Streptomycin (1g IM twice daily) is an effective alternative therapy. * **Lymphogranuloma venereum (LGV):** Caused by *Chlamydia trachomatis* (serotypes L1-L3). The treatment of choice is **Doxycycline** or Erythromycin. Aminoglycosides like streptomycin are ineffective against intracellular *Chlamydia*. * **Syphilis:** Caused by *Treponema pallidum*. The drug of choice is **Benzathine Penicillin G**. Streptomycin has no role in treating spirochetal infections. * **Chancroid:** Caused by *Haemophilus ducreyi* (painful ulcers). The preferred treatments are **Azithromycin**, Ceftriaxone, or Ciprofloxacin. **High-Yield Clinical Pearls for NEET-PG:** * **Streptomycin Uses:** Remember the mnemonic **"T-P-G-H"**: **T**uberculosis (1st line), **P**lague (*Yersinia pestis* - Drug of Choice), **G**ranuloma venereum, and **H**uman Tularemia. * **Adverse Effects:** It is the most vestibulotoxic aminoglycoside (causing vertigo and nystagmus) and is contraindicated in pregnancy due to the risk of fetal ototoxicity. * **Mechanism:** It inhibits bacterial protein synthesis by binding to the **30S ribosomal subunit**, causing misreading of mRNA.

Q: Which one of the following statements about doxycycline is false?

It is more active than tetracycline against H. pylori. **Explanation:** The correct answer is **C**. While doxycycline is a versatile tetracycline, **tetracycline (HCl)** is specifically preferred over doxycycline in the standard quadruple therapy for *H. pylori* eradication. Clinical studies show that tetracycline achieves better mucosal concentrations and efficacy against *H. pylori* compared to doxycycline. **Analysis of Options:** * **A. It is bacteriostatic:** Like all tetracyclines, doxycycline inhibits protein synthesis by binding to the **30S ribosomal subunit**, preventing the attachment of aminoacyl-tRNA. This action is reversible and inhibits growth rather than killing the bacteria. * **B. It is excreted mainly in the feces:** Doxycycline is unique among tetracyclines because it is primarily excreted into the gut via bile and eliminated in feces. This makes it the **tetracycline of choice in patients with renal failure**, as it does not accumulate in the blood. * **D. It is used in Lyme's disease:** Doxycycline is the **first-line drug of choice** for early-stage Lyme disease (*Borrelia burgdorferi*), as well as for Rickettsial infections and Chlamydia. **High-Yield Clinical Pearls for NEET-PG:** * **Phototoxicity:** Doxycycline is the most common tetracycline to cause skin photosensitivity. * **Contraindications:** Avoid in pregnancy and children <8 years due to permanent tooth discoloration and bone growth retardation. * **Absorption:** Unlike older tetracyclines, doxycycline absorption is **not** significantly affected by food or milk. * **Prophylaxis:** It is used for the prophylaxis of Malaria (in mefloquine-resistant areas) and Leptospirosis.

Question 1

Which ATT drug causes contact lens staining?

Accepted Answer

Rifampicin

Answer

Isoniazid (INH)

Answer

Streptomycin

Answer

Pyrazinamide

Question 2

Once weekly administration of which of the following antibiotics has prophylactic activity against bacteraemia caused by M.avium complex in AIDS patients?

Accepted Answer

Azithromycin

Answer

Clarithromycin

Answer

Isoniazid

Answer

Rifabutin

Question 3

Which of the following drug toxicities can be seen in Crigler-Najjar syndrome?

Accepted Answer

Atazanavir

Answer

Tenofovir

Answer

Zidovudine

Answer

Nevirapine

Question 4

What is the drug of choice for prophylaxis of tuberculosis?

Accepted Answer

Isoniazid

Answer

Rifampicin

Answer

Pyrazinamide

Answer

Streptomycin

Question 5

Which of the following anti-TB drugs is NOT bactericidal?

Accepted Answer

Ethambutol

Answer

Rifampicin

Answer

Isoniazid

Answer

Pyrazinamide

Question 6

What is the drug of choice for MRSA skin infection?

Accepted Answer

Vancomycin

Answer

Streptogramin

Answer

Quinupristin

Answer

Linezolid

Question 7

Which of the following drugs can be used for the treatment of chloroquine-resistant malaria in children?

Accepted Answer

Clindamycin

Answer

Chloroquine

Answer

Doxycycline

Answer

Tetracycline

Question 8

Pyrantel pamoate is effective in which of the following parasitic infections?

Accepted Answer

Enterobius and ascariasis

Answer

Amoebiasis and trichuriasis

Answer

Amoebiasis and strongyloides

Answer

Taenia solium and ascariasis

Question 9

Streptomycin is useful in the treatment of which of the following conditions?

Accepted Answer

Granuloma venereum

Answer

Lymphogranuloma venereum (LGV)

Answer

Syphilis

Answer

Chancroid

Question 10

Which one of the following statements about doxycycline is false?

Accepted Answer

It is more active than tetracycline against H. pylori

Answer

It is bacteriostatic

Answer

It is excreted mainly in the feces

Answer

It is used in Lyme's disease

Antimicrobial Agents — MCQs

Antimicrobial Agents — MCQs

On this page

Practice by Chapter

Want unlimited practice?