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Difficulty: HardPharmacologic Interventions

A 31-year-old pregnant woman (G2P1) at 34 weeks gestation arrives at the obstetric triage unit reporting severe right-sided back pain, nausea, and shaking chills over the past 24 hours. Her temperature is 39.1°C (102.4°F), blood pressure is 100/60 mm Hg, pulse is 112/min, and respiratory rate is 20/min. Physical examination reveals pronounced right costovertebral angle tenderness. Fetal heart rate tracing is reassuring, and there are no uterine contractions. Laboratory studies show a leukocyte count of 16,500/mm³. Urinalysis is positive for nitrites, leukocyte esterase, and >50 white blood cells per high-power field. The patient's medical history is notable for a severe anaphylactic reaction characterized by hypotension and wheezing following amoxicillin administration during childhood. Five years ago, she developed a diffuse maculopapular rash and facial swelling after taking cephalexin. Which of the following is the most appropriate empiric pharmacotherapy for this patient?

  1. Intravenous aztreonamAnswer
  2. B
    Intravenous ciprofloxacin
  3. C
    Intravenous meropenem
  4. D
    Intravenous gentamicin
  5. E
    Oral nitrofurantoin

Answer

Intravenous aztreonam is the most appropriate empiric pharmacotherapy because it provides necessary gram-negative coverage for pyelonephritis, is safe in pregnancy, and will not trigger cross-reactivity in a patient with severe beta-lactam anaphylaxis.
The correct answer is intravenous aztreonam. This patient presents with acute pyelonephritis during the third trimester of pregnancy, a serious infection that requires prompt hospital admission and intravenous antibiotics due to the high risk of maternal sepsis and preterm labor. The standard first-line therapies for pyelonephritis in pregnancy typically include intravenous broad-spectrum beta-lactams, such as ceftriaxone or ampicillin-sulbactam. However, this patient has a history of true, severe IgE-mediated anaphylaxis to amoxicillin (hypotension, wheezing) and a severe reaction to cephalexin (facial swelling/angioedema). Consequently, all penicillins and cephalosporins are absolutely contraindicated. While carbapenems have a low rate of cross-reactivity, they are generally avoided in patients with a history of life-threatening beta-lactam anaphylaxis unless no other options exist. Aztreonam is a monobactam antibiotic that targets gram-negative aerobic bacteria (the primary culprits in pyelonephritis, such as E. coli). Critically, its monocyclic beta-lactam ring structure confers no cross-reactivity with penicillins or cephalosporins (with the sole exception of ceftazidime, which shares an identical side chain). Aztreonam is highly effective for pyelonephritis and has an established safety profile in pregnancy, making it the most appropriate choice.

Step-by-Step Solution

1
Diagnose the acute condition based on the clinical presentation.
The patient has acute pyelonephritis, presenting with fever, flank pain, costovertebral angle tenderness, and a suggestive urinalysis in the third trimester of pregnancy.
Establishing the diagnosis determines the required scope of antimicrobial coverage (primarily enteric gram-negative rods like E. coli) and the need for systemic, tissue-penetrating pharmacokinetics.
2
Determine the appropriate setting and route of administration.
Hospital admission and intravenous antibiotics are required.
Acute pyelonephritis in pregnancy carries a high risk of progression to maternal sepsis, acute respiratory distress syndrome (ARDS), and preterm labor. Rapid systemic stabilization via the IV route is mandatory.
3
Analyze the patient's allergy history to rule out contraindicated classes.
Penicillins, cephalosporins, and carbapenems must be avoided due to the history of IgE-mediated anaphylaxis (hypotension, wheezing, facial swelling).
Administering a beta-lactam with cross-reactivity potential in a patient with documented anaphylaxis risks a fatal hypersensitivity reaction.
4
Select a pharmacologic agent that safely balances antimicrobial efficacy and fetal safety.
Aztreonam is selected.
Aztreonam, a monobactam, provides isolated gram-negative coverage suitable for pyelonephritis, has no structural cross-reactivity with penicillins/cephalosporins (except ceftazidime), and is safe during pregnancy, unlike fluoroquinolones or aminoglycosides.

Key Concept

Empiric antimicrobial selection in pregnant patients with severe beta-lactam allergy
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Topics

Pharmacologic Interventions

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