A 23-year-old woman presents with progressively worsening headache, photophobia, and intermittent fever that have lasted for 6 days. She says her headache is mostly frontal and radiates down her neck. She denies any recent history of blood transfusions, recent travel, or contact with animals. Her past medical history is unremarkable. She is sexually active with a single partner for the past 3 years. Her temperature is 38.5°C (101.3°F). On physical examination, she appears pale and diaphoretic. A fine erythematous rash is noted on the neck and forearms. A lumbar puncture is performed and CSF analysis reveals: Opening pressure: 300 mm H2O Erythrocytes: None Leukocytes: 72/mm3 Neutrophils: 10% Lymphocytes: 75% Mononuclear: 15% Protein: 100 mg/dL Glucose: 70 mg/dL Which of the following is the most likely diagnosis in this patient?

Lymphocytic choriomeningitis virus

A 29-year-old man presents to the clinic complaining of fatigue and loss of sensation in his lower legs. The patient notes no history of trauma or chronic disease but states that he spends a lot of time outside and often encounters wild animals. On examination, the patient has multiple dark lesions over the skin of his face and back, as well as a decreased sensation of fine touch and vibration bilaterally in the lower extremities. What is the morphology of the etiologic agent causing this patient’s symptoms?

An acid-fast, intracellular bacillus

Reactivation of latent viral infection

Maltose-fermenting gram-negative diplococci

A spirochete transmitted via tick

Gram-positive, branching anaerobe

A 24-year-old man presents with a painless genital ulcer for the past 2 weeks. He reports that he recently has been having unprotected sex with multiple partners. Past medical history is unremarkable. On physical examination, a single ulcer is present on the dorsal shaft of the penis which is circumscribed, indurated, and partially healed. There is moderate inguinal lymphadenopathy but no buboes. Which of the following tests would confirm the most likely diagnosis in this patient?

Perform a darkfield microscopic examination of a swab from the chancre

Swab the chancre and perform a saline wet mount

Fluorescent treponemal antibody absorption (FTA-ABS) test

Venereal Disease Research Laboratory (VDRL) test

Leptospira species for USMLE Step 2 CK: High-Yield Microbiology Lessons

Leptospira - The Surfer's Spirochete

Leptospira species under darkfield microscopy

Organism: Spirochete with fine coils and hooked ends, resembling a question mark.
Transmission: Zoonotic; acquired through contact with water or soil contaminated with urine from infected animals (rodents, dogs).
- High risk: Surfers, triathletes, sewer workers.
Clinical: Biphasic illness.
- Phase 1 (Leptospiremic): Flu-like symptoms, myalgias (esp. calves), and pathognomonic conjunctival suffusion.
- Phase 2 (Immune): Weil's disease (severe form).

⭐ Weil's Disease: Characterized by a triad of icterus (jaundice), hemorrhage, and acute kidney injury.

Epidemiology & Transmission - Pond Scum Peril

Zoonosis: Endemic in tropical/subtropical areas. Reservoir is the urine of infected animals (rodents, dogs, livestock).
Transmission: Occurs via contact with contaminated water or soil.
- Spirochetes penetrate mucous membranes or abraded skin.
High-Risk Groups:
- Farmers, veterinarians, sewer workers.
- Recreational exposure: swimming, rafting, or kayaking in freshwater.

⭐ A classic vignette involves a surfer in Hawaii presenting with fever, myalgias, and conjunctival suffusion.

Leptospirosis transmission cycle and human manifestations

Clinical Presentation - The Biphasic Breakdown

Leptospirosis classically presents in two phases after a 1-2 week incubation.

Phase 1 (Septicemic): Abrupt onset.
- High fever, chills, intense headache.
- Severe myalgias, classically in calves & lumbar area.
- Conjunctival suffusion (redness without pus) is pathognomonic.
Phase 2 (Immune): Antibody production begins.
- Recurrence of fever.
- Aseptic meningitis is a common feature.
- Can progress to severe disease (Weil's Disease).

Conjunctival suffusion and jaundice in Weil's disease

⭐ Weil's Disease: The severe, icteric form characterized by a triad of jaundice, azotemia (renal failure), and hemorrhagic manifestations (e.g., pulmonary hemorrhage).

Diagnosis & Treatment - Lab Sleuthing & Bug Busting

Diagnosis:
- Serology (Gold Standard): Microscopic Agglutination Test (MAT) showing a ≥4-fold rise in titer.
- Microscopy: Darkfield microscopy for direct visualization in blood/urine (first week).
- Culture: Slow; requires special media (e.g., Fletcher's).
- PCR: Rapid detection in early stages.
Treatment:
- Mild/Anicteric: Doxycycline, Azithromycin.
- Severe/Icteric (Weil's Disease): IV Penicillin G, Ceftriaxone.

⭐ A Jarisch-Herxheimer reaction (fever, rigors, hypotension) may occur after starting antibiotics due to endotoxin release from killed spirochetes.

High‑Yield Points - ⚡ Biggest Takeaways

Leptospira is a thin, motile spirochete with characteristic hooked ends, typically transmitted through water contaminated with animal urine.

Presents as a biphasic illness: an initial septicemic phase (fever, myalgias) followed by an immune phase.

Weil's disease is the severe form, classically defined by a triad of jaundice, renal failure, and hemorrhage.

A key physical finding is conjunctival suffusion (redness without inflammatory exudate).

Primarily treated with doxycycline or penicillin G.

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