A 29-year-old woman is brought to the emergency department after an episode of syncope. For the past 10 days, she has had dyspnea and palpitations occurring with mild exertion. The patient returned from a hiking trip in Upstate New York 5 weeks ago. Except for an episode of flu with fever and chills a month ago, she has no history of serious illness. Her temperature is 37.3°C (99.1°F), pulse is 45/min, respirations are 21/min, and blood pressure is 148/72 mm Hg. A resting ECG is shown. Two-step serological testing confirms the diagnosis. Which of the following is the most appropriate treatment?

Permanent pacemaker implantation

A 28-year-old man presents to the clinic complaining of chronic joint pain and fatigue for the past 2 months. The patient states that he usually has pain in one of his joints that resolve but then seems to move to another joint. The patient notes no history of trauma but states that he has experienced some subjective fevers over that time. He works as a logger and notes that he's heard that people have also had these symptoms in the past, but that he does not know anyone who is currently experiencing them. What is the most likely etiologic agent of this patient's disease?

A gram-positive, spore-forming rod

ssDNA virus of the Parvoviridae family

A gram-positive cocci in chains

A 29-year-old woman presents with a skin rash that has spread on her arm over the last few days. She also complains of fever, headache, joint pain, and stiffness of the neck associated with the onset of the rash. On physical examination, there is an annular, red rash with a clear area in the center similar to a bull’s-eye (see image). The patient says she went on a camping trip to Connecticut last month but does not remember being bitten by an insect. Which of the following could result if this condition remains untreated in this patient?

Subacute sclerosing panencephalitis

Blood cultures are sent to the laboratory and empiric treatment with intravenous vancomycin is started. Blood cultures grow gram-negative bacilli identified as Cardiobacterium hominis. Which of the following is the most appropriate next step in management?

Switch to intravenous ceftriaxone

Switch to intravenous gentamicin

Switch to intravenous ampicillin

Switch to intravenous cefazolin

Borrelia species for USMLE Step 1: High-Yield Microbiology Lessons

Borrelia - The Great Imitator

Organism: Spirochete (spiral-shaped bacteria); visualized via darkfield microscopy or Wright/Giemsa stains.
Transmission: Tick-borne, primarily by Ixodes ticks.
Lyme Disease (B. burgdorferi):
- Stage 1: Erythema migrans (bull's-eye rash).
- Stage 2: Facial nerve (Bell's) palsy, AV block.
- Stage 3: Chronic arthritis, encephalopathy.
Relapsing Fever (B. recurrentis): Recurrent fevers due to antigenic variation.
Treatment: Doxycycline, amoxicillin, or ceftriaxone.

⭐ A key diagnostic challenge is that the classic bull's-eye rash is absent in ~20% of Lyme disease cases.

Varied presentations of Lyme disease rashes

Lyme Disease - A Tick-ing Time Bomb

Organism: Borrelia burgdorferi (spirochete).
Vector: Ixodes scapularis (deer tick).
Reservoir: White-footed mouse.

Lyme disease rash variations and common symptoms

Clinical Stages & Presentation

⭐ Co-infection is common with Babesia or Anaplasma phagocytophilum as they share the same Ixodes tick vector.

Diagnosis & Treatment

Dx: Clinical (rash); Serology (ELISA, Western blot) confirms later stages.
Tx: Doxycycline. Amoxicillin for children <8 or pregnant patients.
📌 Mnemonic: "BAKE a Key Lyme pie":
- Bell's Palsy
- Arthritis
- Kardiac block
- Erythema migrans

Relapsing Fever - The Comeback King

Borrelia Spirochetes in Giemsa-stained Blood Smear

Organism: Borrelia spp. (B. recurrentis, B. hermsii).
Transmission:
- Louse-borne (Epidemic): B. recurrentis (human body louse).
- Tick-borne (Endemic): B. hermsii (soft Ornithodoros tick).
Pathogenesis: Antigenic variation of surface proteins causes immune evasion and relapsing fevers.
Presentation: Recurrent cycles of abrupt high fever, headache, and myalgia, separated by afebrile intervals.
Diagnosis: Visible spirochetes on peripheral blood smear (Giemsa/Wright stain) during febrile periods.
Treatment: Doxycycline. ⚠️ High risk of Jarisch-Herxheimer reaction.

⭐ The relapsing course of fever is directly due to the spirochete's ability to switch its variable major proteins (VMPs), creating new antigenic types that evade the host's antibodies.

Lab & Pharmacy - A Tale of Two Fevers

Feature	Lyme Disease (B. burgdorferi)	Relapsing Fever (B. recurrentis)
Diagnosis	Serology is key (ELISA, Western blot). Spirochetes are not seen on smear.	Microscopy of peripheral blood smear (Wright or Giemsa stain) shows large spirochetes.
Treatment	Doxycycline is first-line. Amoxicillin or cefuroxime for children <8 or pregnant women.	Doxycycline or tetracycline.
Advanced Rx	IV Ceftriaxone for carditis or neurologic disease (meningitis, encephalitis).	Manage Jarisch-Herxheimer reaction.

High‑Yield Points - ⚡ Biggest Takeaways

Borrelia burgdorferi, a spirochete, causes Lyme disease and is transmitted by the Ixodes tick.

Primary stage is marked by Erythema migrans, a characteristic bull's-eye rash.

Secondary stage features include bilateral facial nerve palsy and AV nodal block.

Tertiary stage presents with migratory monoarthritis of large joints and subtle encephalopathy.

Diagnosis is clinical, confirmed by a two-tier serology: ELISA then Western blot.

Treat with Doxycycline; use Ceftriaxone for late-stage or severe neurologic/cardiac manifestations.

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