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Difficulty: MediumInfectious Diseases

Select the most appropriate first-line antimicrobial therapy for each of the following patients presenting with a characteristic infectious syndrome.

  • A 34-year-old patient presenting with an expanding 8-cm annular erythematous rash with central clearing on the thigh after a recent camping trip.Oral doxycycline
  • A 22-year-old patient presenting with acute dysuria, urinary frequency, and suprapubic discomfort, with no fever or costovertebral angle tenderness.Oral nitrofurantoin
  • A 55-year-old patient presenting with a rapidly spreading, poorly demarcated, warm, tender area of erythema on the lower leg without purulent drainage.Oral cephalexin
  • A 30-year-old patient presenting with a single 1-cm painless, indurated anogenital ulcer with a clean base and non-tender inguinal lymphadenopathy.Intramuscular benzathine penicillin G

Answer

The correct matches pair early localized Lyme disease with oral doxycycline, uncomplicated cystitis with oral nitrofurantoin, non-purulent cellulitis with oral cephalexin, and primary syphilis with intramuscular benzathine penicillin G.
Each specific clinical vignette describes a hallmark presentation of a common outpatient infection, each with a highly standardized first-line treatment pathway. Recognizing erythema migrans directs the clinician to doxycycline; uncomplicated dysuria directs to nitrofurantoin; non-purulent skin erythema directs to a first-generation cephalosporin like cephalexin; and a painless genital chancre strictly mandates benzathine penicillin G.

Step-by-Step Solution

1
Evaluate the first presentation.
Diagnose early localized Lyme disease based on the classic erythema migrans rash.
Identifying the disease allows for the selection of the correct first-line therapy, which is oral doxycycline.
2
Evaluate the second presentation.
Diagnose uncomplicated acute cystitis due to the presence of lower urinary tract symptoms without systemic signs.
Nitrofurantoin is first-line because it concentrates effectively in the urine and avoids unnecessarily broad systemic coverage.
3
Evaluate the third presentation.
Diagnose non-purulent cellulitis based on the rapidly spreading erythema without an abscess or purulence.
Oral cephalexin is selected as it provides optimal coverage against beta-hemolytic streptococci and methicillin-susceptible Staphylococcus aureus (MSSA).
4
Evaluate the fourth presentation.
Diagnose primary syphilis based on the single painless genital chancre.
Intramuscular benzathine penicillin G is the definitive treatment for all stages of syphilis (unless neurosyphilis is present).

Key Concept

First-line empiric and targeted antimicrobial selection for classic outpatient infectious syndromes.
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Topics

Infectious Diseases

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