Antimicrobial Agents Practice Questions

Q: Resistance to zidovudine develops due to:

Mutation at reverse transcriptase. ***Mutation at reverse transcriptase*** - Zidovudine is a **nucleoside reverse transcriptase inhibitor (NRTI)** that acts by inhibiting **HIV reverse transcriptase**. - **Mutations in the reverse transcriptase gene** can alter the enzyme's structure, preventing zidovudine from binding effectively or allowing the enzyme to efficiently excise the incorporated zidovudine, thereby leading to drug resistance. *Increased efflux of the drug from inside the cell* - While efflux pumps can cause drug resistance in some contexts (e.g., cancer chemotherapy), they are **not the primary mechanism** of zidovudine resistance in HIV. - Zidovudine resistance is predominantly driven by **viral enzyme modifications**. *Increased metabolism of the drug* - Enhanced host metabolism of zidovudine would reduce its availability, but this is **not a primary mechanism** of virus-mediated resistance. - Viral resistance mechanisms typically involve alterations in the **target protein** (reverse transcriptase) or reduced drug activation. *Decreased zidovudine 5 triphosphate formation* - Zidovudine must be **phosphorylated intracellularly to its active triphosphate form** to inhibit reverse transcriptase. - While impaired phosphorylation could theoretically reduce drug efficacy, zidovudine resistance is predominantly linked to **reverse transcriptase mutations** rather than issues with host cell phosphorylation.

Q: Which of the following is not an antifungal drug?

Clofazimine. ***Clofazimine*** - **Clofazimine** is an **antibiotic** primarily used in the treatment of **leprosy** and occasionally other mycobacterial infections. - It does not have significant antifungal activity and its mechanism involves binding to bacterial DNA and inhibiting growth. *Ketoconazole* - **Ketoconazole** is an **azole antifungal** drug commonly used to treat various fungal infections, including dermatomycoses and systemic candidiasis. - Its mechanism of action involves inhibiting **ergosterol synthesis**, a vital component of fungal cell membranes. *Undecylenic acid* - **Undecylenic acid** is a topical **fatty acid antifungal** derived from castor bean oil, used for superficial fungal infections, especially ringworm. - It works by disrupting the **cell membranes** of fungi, leading to leakage of cellular contents. *Ciclopirox* - **Ciclopirox** is a broad-spectrum **hydroxypyridone antifungal** agent used primarily for topical treatment of dermatophyte, yeast, and other fungal skin infections, as well as onychomycosis. - Its mechanism involves inhibiting the transport of essential substances into the fungal cell and interfering with **DNA, RNA, and protein synthesis**.

Q: Most important side effect of ethambutol is:

Retrobulbar neuritis. ***Retrobulbar neuritis*** - **Ethambutol** is known to cause **retrobulbar neuritis**, which presents as **decreased visual acuity**, **red-green color blindness**, and a **central scotoma** [1, 2]. - This side effect is typically **dose-dependent** [1] and can be reversible if detected early and the drug is discontinued. *Hepatotoxicity* - While other anti-tuberculosis drugs like **isoniazid** and **rifampin** are commonly associated with hepatotoxicity, it is **not the primary or most important side effect of ethambutol**. - Ethambutol's impact on the liver is generally minor compared to its ocular effects. *Renal toxicity* - **Renal toxicity** is **not a significant or common side effect** of ethambutol. - Patients with pre-existing renal impairment may require dose adjustments due to altered drug elimination, but the drug itself does not commonly induce renal damage. *Peripheral neuropathy* - **Peripheral neuropathy** is a well-known side effect of **isoniazid**, another anti-tuberculosis drug, particularly in patients with pre-existing risk factors or vitamin B6 deficiency. - It is **rarely associated with ethambutol** and is not considered its primary adverse effect.

Q: Which of the following is a first-generation cephalosporin used for surgical prophylaxis?

Cefazolin. ***Cefazolin*** - **Cefazolin** is a **first-generation cephalosporin** routinely used for **surgical prophylaxis** due to its effective coverage against common skin flora like *Staphylococcus aureus* and streptococci. - Its **longer half-life** allows for less frequent dosing pre-operatively, making it practical for preventing surgical site infections. *Ceftriaxone* - **Ceftriaxone** is a **third-generation cephalosporin** with a broader spectrum of activity, including good coverage against many gram-negative bacteria, but it is not typically the first choice for routine surgical prophylaxis. - It is more commonly reserved for treating serious infections such as **meningitis**, **gonorrhea**, and complicated intra-abdominal infections. *Cefoxitin* - **Cefoxitin** is a **second-generation cephalosporin** known for its excellent activity against **anaerobic bacteria**, in addition to gram-positive and some gram-negative organisms. - While it can be used for surgical prophylaxis in procedures with **high anaerobic risk** (e.g., colorectal surgery), it is not a first-generation cephalosporin. *Cefepime* - **Cefepime** is a **fourth-generation cephalosporin** with a very broad spectrum of activity, including excellent coverage against **Pseudomonas aeruginosa** and improved activity against gram-positive bacteria compared to third-generation cephalosporins. - It is reserved for severe infections, such as **febrile neutropenia** and hospital-acquired pneumonia, and is not generally used for routine surgical prophylaxis.

Q: What is the drug of choice for treating Mycoplasma infections?

Azithromycin. ***Azithromycin*** - **Azithromycin** is a macrolide antibiotic, which is highly effective against **Mycoplasma species** due to its mechanism of inhibiting bacterial protein synthesis. - **Mycoplasma** lacks a cell wall, rendering beta-lactam antibiotics ineffective; macrolides like azithromycin are therefore a primary choice. *Cefadroxil* - **Cefadroxil** is a **cephalosporin antibiotic** (a type of beta-lactam), which targets bacterial cell wall synthesis. - **Mycoplasma** species intrinsically lack a cell wall, making them resistant to all beta-lactam antibiotics, including cefadroxil. *Erythromycin* - **Erythromycin** is an older macrolide antibiotic that, while effective against Mycoplasma, has a **less favorable pharmacokinetic profile** (e.g., more frequent dosing) and often more gastrointestinal side effects compared to newer macrolides like azithromycin. - **Azithromycin** is generally preferred due to its **once-daily dosing** and better tolerability, making it the more common "drug of choice" among macrolides for Mycoplasma infections. *Chloramphenicol* - **Chloramphenicol** is a broad-spectrum antibiotic that inhibits bacterial protein synthesis, but it is typically reserved for **life-threatening infections** due to its potential for serious side effects, such as **bone marrow suppression** (aplastic anemia). - While active against Mycoplasma, its toxicity profile makes it **not a first-line drug of choice** when safer and equally effective alternatives like macrolides are available.

Q: Which of the following bacteria are not affected by streptogramins?

E. coli. ***E. coli*** - Streptogramins (e.g., quinupristin/dalfopristin) are primarily active against **Gram-positive bacteria**. - ***E. coli*** is a **Gram-negative bacterium** with intrinsic resistance to streptogramins due to poor penetration through the outer membrane. - This makes E. coli the most clearly **NOT affected** by streptogramins among the options. *Staphylococcus aureus* - **Streptogramins** are highly effective against **Gram-positive bacteria**, including **Staphylococcus aureus**. - They are particularly useful for treating **multidrug-resistant strains like MRSA** and vancomycin-resistant Enterococcus faecium. - This is one of their primary clinical indications. *Legionella* - **_Legionella_** species are **atypical intracellular bacteria** that cause Legionnaires' disease. - While streptogramins have some intracellular penetration, they have **limited and unreliable activity** against Legionella. - Standard treatment uses macrolides or fluoroquinolones, NOT streptogramins. *M. pneumoniae* - **_Mycoplasma pneumoniae_** is an **atypical bacterium** lacking a cell wall that causes atypical pneumonia. - Streptogramins have **limited clinical activity** against M. pneumoniae and are NOT used as standard therapy. - Preferred treatments include macrolides, tetracyclines, or fluoroquinolones.

Q: Which antibiotic acts by inhibiting protein synthesis?

Doxycycline. ***Doxycycline*** - **Doxycycline** is a **tetracycline antibiotic** that inhibits bacterial protein synthesis by binding reversibly to the **30S ribosomal subunit**, preventing the attachment of aminoacyl-tRNA. - This action effectively blocks the elongation of the peptide chain, halting bacterial growth. *Cefotetan* - **Cefotetan** is a **second-generation cephalosporin** that inhibits bacterial cell wall synthesis. - It does this by binding to and inactivating **penicillin-binding proteins (PBPs)**, which are essential for peptidoglycan cross-linking. *Ciprofloxacin* - **Ciprofloxacin** is a **fluoroquinolone antibiotic** that inhibits bacterial **DNA synthesis**. - It targets **DNA gyrase (topoisomerase II)** and **topoisomerase IV**, enzymes crucial for DNA replication, transcription, and repair. *Oxacillin* - **Oxacillin** is a **beta-lactam antibiotic** (specifically a penicillinase-resistant penicillin) that, like cephalosporins, inhibits bacterial **cell wall synthesis**. - It achieves this by binding to and inactivating **penicillin-binding proteins (PBPs)**, thereby interfering with peptidoglycan cross-linking.

Q: Which of the following drugs causes flaccid paralysis of ascaris?

Piperazine. ***Piperazine*** - Piperazine acts as a **GABA agonist**, leading to **hyperpolarization** of the *Ascaris* muscle cells. - This hyperpolarization causes muscular paralysis, making the worms unable to maintain their position and thus they are expelled. *Albendazole* - Albendazole primarily works by inhibiting **microtubule polymerization** in the parasitic worms, which impairs glucose uptake and energy metabolism. - This leads to the **slow death** of the worms, rather than acute flaccid paralysis. *Pyrantel pamoate* - Pyrantel pamoate induces a **nicotinic acetylcholine receptor agonism** in nematodes, causing **spastic paralysis** [1]. - The worms become irreversibly paralyzed in a contracted state, unlike the flaccid paralysis caused by piperazine. *Ivermectin* - Ivermectin works by binding to **glutamate-gated chloride channels** in invertebrate nerve and muscle cells. - This binding leads to an increase in chloride ion permeability and **hyperpolarization** of the neuronal membrane, causing paralysis and eventual death of the parasite.

Q: Which of the following drugs is not effective against methicillin-resistant Staphylococcus aureus (MRSA)?

Cefaclor. ***Cefaclor*** - **Cefaclor** is a second-generation cephalosporin, and like most beta-lactam antibiotics, it is **ineffective against MRSA** due to the presence of the **mecA gene** which codes for an altered penicillin-binding protein (PBP2a). - MRSA's resistance mechanism renders beta-lactams, including cefaclor, unable to bind effectively to their target, thus inhibiting their antibacterial action. *Cotrimoxazole* - **Cotrimoxazole** (trimethoprim-sulfamethoxazole) is often effective against MRSA, especially for **skin and soft tissue infections**, making it a common choice for outpatient treatment. - It works by inhibiting two consecutive steps in the bacterial **folic acid synthesis pathway**, preventing nucleic acid and protein synthesis. *Ciprofloxacin* - **Ciprofloxacin**, a fluoroquinolone, can be effective against certain strains of MRSA, although **resistance rates have increased**, limiting its broad utility. - Its mechanism involves inhibiting **bacterial DNA gyrase** and topoisomerase IV, essential enzymes for DNA replication. *Vancomycin* - **Vancomycin** is a cornerstone in the treatment of **severe MRSA infections**, particularly in intravenous formulations for systemic infections. - It inhibits **bacterial cell wall synthesis** by binding to the D-Ala-D-Ala terminus of peptidoglycan precursors.

Q: All the following statements are true regarding beta-lactams except:

Aztreonam shows cross-reactivity with cephalexin.. ***Aztreonam shows cross-reactivity with cephalexin.*** - **Aztreonam** is a monobactam with a distinct chemical structure from other beta-lactams, resulting in a different **allergy profile** and lack of significant **cross-reactivity** with other beta-lactams, including cephalosporins like cephalexin. - Its unique structure also means it is specifically active against **gram-negative bacteria** and generally resistant to common beta-lactamases that deactivate other beta-lactams. *Imipenem should be given with cilastatin* - **Imipenem** is rapidly metabolized in the renal tubules by **dehydropeptidase-1**, leading to reduced antibiotic concentrations and potential for nephrotoxicity. - **Cilastatin** is a dehydropeptidase inhibitor that prevents the breakdown of imipenem, ensuring adequate drug levels and reducing renal damage. *Methicillin is not orally bioavailable and is given parenterally.* - **Methicillin** is an acid-labile penicillin, meaning it is extensively degraded by stomach acid when taken orally, leading to poor absorption. - Due to its instability in acidic environments, methicillin must be administered **parenterally (intravenously or intramuscularly)** to achieve therapeutic concentrations. *Meropenem does not require cilastatin for protection against renal toxicity.* - **Meropenem** is another carbapenem, but it is much more stable to metabolism by **renal dehydropeptidase-1** compared to imipenem. - This inherent stability eliminates the need for co-administration with **cilastatin** to prevent its degradation and protect against nephrotoxicity.

Question 1

Resistance to zidovudine develops due to:

Accepted Answer

Mutation at reverse transcriptase

Answer

Increased efflux of the drug from inside the cell

Answer

Increased metabolism of the drug

Answer

Decreased zidovudine 5'-triphosphate formation

Question 2

Which of the following is not an antifungal drug?

Accepted Answer

Clofazimine

Answer

Undecylenic acid

Answer

Ciclopirox

Answer

Ketoconazole

Question 3

Most important side effect of ethambutol is:

Accepted Answer

Retrobulbar neuritis

Answer

Hepatotoxicity

Answer

Renal toxicity

Answer

Peripheral neuropathy

Question 4

Which of the following is a first-generation cephalosporin used for surgical prophylaxis?

Accepted Answer

Cefazolin

Answer

Ceftriaxone

Answer

Cefoxitin

Answer

Cefepime

Question 5

What is the drug of choice for treating Mycoplasma infections?

Accepted Answer

Azithromycin

Answer

Cefadroxil

Answer

Erythromycin

Answer

Chloramphenicol

Question 6

Which of the following bacteria are not affected by streptogramins?

Accepted Answer

E. coli

Answer

Legionella

Answer

Staphylococcus aureus

Answer

M. pneumoniae

Question 7

Which antibiotic acts by inhibiting protein synthesis?

Accepted Answer

Doxycycline

Answer

Cefotetan

Answer

Ciprofloxacin

Answer

Oxacillin

Question 8

Which of the following drugs causes flaccid paralysis of ascaris?

Accepted Answer

Piperazine

Answer

Ivermectin

Answer

Albendazole

Answer

Pyrantel pamoate

Question 9

Which of the following drugs is not effective against methicillin-resistant Staphylococcus aureus (MRSA)?

Accepted Answer

Cefaclor

Answer

Ciprofloxacin

Answer

Cotrimoxazole

Answer

Vancomycin

Question 10

All the following statements are true regarding beta-lactams except:

Accepted Answer

Aztreonam shows cross-reactivity with cephalexin.

Answer

Methicillin is not orally bioavailable and is given parenterally.

Answer

Imipenem should be given with cilastatin

Answer

Meropenem does not require cilastatin for protection against renal toxicity.

Antimicrobial Agents — MCQs

Antimicrobial Agents — MCQs

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