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Difficulty: Very hardInfectious Diseases

A 72-year-old man is evaluated on hospital day 3 for discharge planning. He was initially admitted with fever, productive cough, and left-sided pleuritic chest pain. Chest radiography on admission demonstrated a dense left lower lobe consolidation. He was started on empiric intravenous ceftriaxone and azithromycin. Two sets of blood cultures drawn upon presentation have grown *Streptococcus pneumoniae*. Antimicrobial susceptibility testing is reported as follows:

AntimicrobialInterpretationMIC (µg/mL)
PenicillinSusceptible0.03
CeftriaxoneSusceptible0.25
ErythromycinResistant>8.0
LevofloxacinSusceptible1.0
VancomycinSusceptible0.5

The patient's clinical status has markedly improved. He has been afebrile for 24 hours, maintains an oxygen saturation of 96% on room air, and is tolerating an oral diet. His medical history includes hypertension and benign prostatic hyperplasia. He has no known drug allergies. The plan is to discharge the patient to complete a total 7-day course of antibiotics at home. Which of the following is the most appropriate oral antibiotic to prescribe for discharge?

  1. A
    Amoxicillin-clavulanate
  2. B
    Levofloxacin
  3. AmoxicillinAnswer
  4. D
    Cefdinir
  5. E
    Linezolid

Answer

Amoxicillin
The patient has community-acquired pneumonia with *Streptococcus pneumoniae* bacteremia. Having achieved clinical stability (afebrile, no oxygen requirement, tolerating oral intake), he is eligible for oral step-down therapy. The susceptibility panel shows the isolate is highly susceptible to penicillin. For penicillin-susceptible *S. pneumoniae*, oral amoxicillin is the drug of choice due to its targeted narrow spectrum, excellent oral bioavailability, and high efficacy in pulmonary tissues. The presence of bacteremia does not require a broader-spectrum agent or an extended intravenous course once the patient demonstrates clinical improvement.

Step-by-Step Solution

1
Identify the primary diagnosis and the patient's current clinical trajectory.
The patient has community-acquired pneumonia with concurrent pneumococcal bacteremia, but is now clinically stable and ready for step-down therapy.
Establishing clinical stability confirms that an oral transition is safe, regardless of initial bacteremia.
2
Analyze the antimicrobial susceptibility data.
The *S. pneumoniae* isolate is highly susceptible to penicillin (MIC 0.03 µg/mL).
Specific MIC values guide the selection of the narrowest effective agent.
3
Recall the resistance mechanism of Streptococcus pneumoniae.
Resistance is mediated by altered penicillin-binding proteins (PBPs), not by beta-lactamase production.
This pathophysiologic mechanism dictates whether a beta-lactamase inhibitor (like clavulanate) is clinically useful.
4
Select the optimal narrow-spectrum oral agent.
Amoxicillin is chosen over amoxicillin-clavulanate, cephalosporins, and fluoroquinolones.
Amoxicillin provides excellent oral bioavailability and targeted coverage without unnecessary ecological disruption.

Key Concept

Oral step-down therapy for susceptible Streptococcus pneumoniae bacteremia and mechanisms of beta-lactam resistance.
Estimated Time:1m 15s
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Topics

Infectious Diseases

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