You are seeing a patient in clinic who recently started treatment for active tuberculosis. The patient is currently being treated with rifampin, isoniazid, pyrazinamide, and ethambutol. The patient is not used to taking medicines and is very concerned about side effects. Specifically regarding the carbohydrate polymerization inhibiting medication, which of the following is a known side effect?

Paresthesias of the hands and feet

An 18-year old college freshman presents to his university clinic because he has not been feeling well for the past two weeks. He has had a persistent headache, occasional cough, and chills without rigors. The patient’s vital signs are normal and physical exam is unremarkable. His radiograph shows patchy interstitial lung infiltrates and he is diagnosed with atypical pneumonia. The patient is prescribed azithromycin and takes his medication as instructed. Despite adherence to his drug regimen, he returns to the clinic one week later because his symptoms have not improved. The organism responsible for this infection is likely resistant to azithromycin through which mechanism?

Methylation of ribosomal binding site

Decreased binding to RNA polymerase

You are a resident in the surgical ICU. One of the patients you are covering is a 35-year-old pregnant G1P0 in her first trimester admitted for complicated appendicitis and awaiting appendectomy. Your attending surgeon would like you to start the patient on moxifloxacin IV preoperatively. You remember from your obstetrics clerkship, however, that moxifloxacin is Pregnancy Category C, and animal studies have shown that immature animals exposed to fluoroquinolones like moxifloxacin may experience cartilage damage. You know that there are potentially safer antibiotics, such as piperacillin/tazobactam, which is in Pregnancy Category B. What should you do?

Wait to administer any antibiotics until you discuss your safety concerns with your attending.

Administer moxifloxacin since it is only Pregnancy Category C and, although studies may have revealed adverse effects in animals, there is no definite evidence that it causes risk in humans.

Administer piperacillin/tazobactam instead of moxifloxacin without discussing with the attending since your obligation is to "first, do no harm" and both are acceptable antibiotics for complicated appendicitis.

Discuss the adverse effects of each antibiotic with the patient, and then let the patient decide which antibiotic she would prefer.

Administer moxifloxacin since the attending is the executive decision maker and had to know the patient was pregnant when deciding on an antibiotic.

You are treating a neonate with meningitis using ampicillin and a second antibiotic, X, that is known to cause ototoxicity. What is the mechanism of antibiotic X?

It binds the 30S ribosomal subunit and inhibits formation of the initiation complex

It binds the 50S ribosomal subunit and inhibits formation of the initiation complex

It binds the 30S ribosomal subunit and reversibly inhibits translocation

It binds the 50S ribosomal subunit and inhibits peptidyltransferase

It binds the 50S ribosomal subunit and reversibly inhibits translocation

A 56-year-old man presents with breathlessness and altered mental status. The patient’s daughter says that he has been having high fever and cough for the last 3 days. Past medical history is significant for a recent hospitalization 5 days ago, following a successful coronary artery bypass grafting (CABG). In the post-operative period, he was in an intensive care unit (ICU) for 6 days, including 12 hours on mechanical ventilation. Current medications are aspirin and rosuvastatin. The patient’s daughter mentions that he has had anaphylactic reactions to penicillin in the past. His temperature is 39.4°C (103°F), pulse rate is 110/min, blood pressure is 104/78 mm Hg, and respiratory rate is 30/min. On physical examination, the patient is confused and disoriented and shows signs of respiratory distress and cyanosis. On chest auscultation, there is crepitus in the right lung. The patient is immediately started on oxygen therapy, intravenous fluids, and supportive care. After the collection of appropriate samples for bacteriological culture, treatment with empirical intravenous antibiotics are started. After 24 hours of treatment, the microbiology results indicate Pseudomonas aeruginosa infection. Antibiotic therapy is changed to a combination of aztreonam and tobramycin. Which of the following best describes the rationale for choosing this antibiotic combination?

Synergism of aztreonam with tobramycin

Broad-spectrum coverage against anaerobes by adding tobramycin to aztreonam

Reduction of the side-effects of both aztreonam and tobramycin

Broad-spectrum coverage against gram-positive cocci by adding tobramycin to aztreonam

Effective combination of a bactericidal and a bacteriostatic antimicrobial against Pseudomonas aeruginosa

Carbapenems and monobactams for USMLE Step 3: High-Yield Pharmacology Lessons

Carbapenems & Monobactams - The Big Guns

Carbapenems (-penem): Broad-spectrum β-lactams, often reserved for multidrug-resistant (MDR) organisms (e.g., ESBL+ bacteria).
- MOA: Inhibit cell wall synthesis via penicillin-binding proteins (PBPs).
- Agents: Imipenem, Meropenem, Ertapenem, Doripenem.
- Adverse: GI distress, rash, CNS toxicity (seizures, esp. Imipenem).
- 💡 Imipenem is always co-administered with Cilastatin (a renal dehydropeptidase I inhibitor) to prevent its degradation.
Monobactams (Aztreonam):
- Spectrum: Narrow; aerobic Gram-negative rods ONLY (including Pseudomonas).

⭐ Aztreonam has no cross-allergenicity with penicillins/cephalosporins, making it safe for most penicillin-allergic patients.

Beta-lactam antibiotic chemical structures

The Carbapenems - 'Penem Powerhouses

Potent, broad-spectrum β-lactam antibiotics often reserved for severe or multidrug-resistant (MDR) infections. They are bactericidal, inhibiting cell wall synthesis by binding to penicillin-binding proteins (PBPs). Their structure makes them highly resistant to most β-lactamases, including ESBLs.

Drug	Gram (+)	Gram (-)	Anaerobes	Pseudomonas	Key Features & Adverse Effects
Imipenem	✅	✅	✅	✅	Must be given with Cilastatin (a dehydropeptidase I inhibitor) to prevent renal inactivation. Highest risk of seizures.
Meropenem	✅	✅	✅	✅	Stable to dehydropeptidase I. Lower seizure risk than imipenem.
Ertapenem	✅	✅	✅	❌	The exception: Lacks coverage for Pseudomonas, Acinetobacter, Enterococcus (📌 APE). Convenient once-daily dosing.
Doripenem	✅	✅	✅	✅	Generally has greater activity against Pseudomonas and is more stable in solution.

Imipenem-Cilastatin - The Kidney Guard

Imipenem: A broad-spectrum carbapenem antibiotic.
Cilastatin: A renal dehydropeptidase I inhibitor; acts as a "bodyguard."
Synergy: Cilastatin prevents renal metabolism of Imipenem, increasing its half-life and protecting against nephrotoxic metabolites.

⚠️ Adverse Effects: CNS toxicity (confusion, myoclonus, seizures), especially in patients with renal insufficiency.

⭐ Imipenem is often nicknamed "seizure-penem." The risk is heightened when co-administered with ganciclovir.

Aztreonam - The Lone Ranger

Class: Monobactam; structurally distinct from other β-lactams.
Mechanism: Binds exclusively to Penicillin-Binding Protein 3 (PBP-3) of Gram-negative aerobes, preventing peptidoglycan cross-linking and inhibiting cell wall synthesis.
Spectrum: Narrow. Active ONLY against aerobic Gram-negative rods, including Pseudomonas aeruginosa.
Key Use: Safe alternative for patients with a history of severe, IgE-mediated penicillin allergy.

⭐ Minimal cross-allergenicity with other β-lactams, with the notable exception of ceftazidime due to a similar R-group side chain.

High‑Yield Points - ⚡ Biggest Takeaways

Carbapenems are broad-spectrum β-lactamase-resistant drugs for multidrug-resistant organisms.

Imipenem is always given with cilastatin, which blocks its inactivation by renal dehydropeptidase I.

A major adverse effect of carbapenems is CNS toxicity, which can lower the seizure threshold.

Aztreonam, a monobactam, has a narrow spectrum limited to aerobic gram-negative rods.

Aztreonam has no cross-allergenicity with penicillins, making it safe for allergic patients.

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