A 53-year-old woman with hypertension is brought to the emergency department 30 minutes after having a generalized, tonic-clonic seizure. She has had recurrent headaches and dizziness in the last 3 months. One year ago, she had diarrhea after a trip to Ecuador that resolved without treatment. She has not received any medical care in the last five years. She has smoked 1 pack of cigarettes daily for 20 years. Her temperature is 36°C (96.8°F) and blood pressure is 159/77mm Hg. Physical examination shows dysarthria and hyperreflexia. She is confused and oriented only to name and place. Four brain lesions are found in a CT scan of the brain; one of the lesions is shown. Which of the following is most likely to have prevented this patient's condition?

Vaccination against meningococcus

Improved blood pressure control

A 35-year-old man is brought to the emergency department after experiencing a seizure. According to his girlfriend, he has had fatigue for the last 3 days and became confused this morning, after which he started having uncontrollable convulsions throughout his entire body. He was unconscious throughout the episode, which lasted about 4 minutes. He has not visited a physician for over 10 years. He has smoked one pack of cigarettes daily for 12 years. His girlfriend admits they occasionally use heroin together with their friends. His temperature is 38.8°C (101.8°F), pulse is 93/min, respirations are 20/min, and blood pressure is 110/70 mm Hg. The lungs are clear to auscultation and examination shows normal heart sounds and no carotid or femoral bruits. He appears emaciated and somnolent. There are multiple track marks on both his arms. He is unable to cooperate for a neurological exam. Laboratory studies show a leukocyte count of 3,000/mm3, a hematocrit of 34%, a platelet count of 354,000/mm3, and an erythrocyte sedimentation rate of 27 mm/h. His CD4+ T-lymphocyte count is 84/mm3 (normal ≥ 500). A CT scan of the head is shown. Which of the following is the most appropriate next step considering this patient's CT scan findings?

Pyrimethamine, sulfadiazine, and leucovorin

CT-guided stereotactic aspiration

Nucleic acid amplification testing (NAAT) of first-void urine confirms infection with Chlamydia trachomatis. Treatment with the appropriate pharmacotherapy is started. Which of the following health maintenance recommendations is most appropriate at this time?

Avoid sexual activity for the next month

Schedule an ophthalmology consultation

A 38-year-old man comes to the physician because of a 2-week history of abdominal pain and an itchy rash on his buttocks. He also has fever, nausea, and diarrhea with mucoid stools. One week ago, the patient returned from Indonesia, where he went for vacation. Physical examination shows erythematous, serpiginous lesions located in the perianal region and the posterior thighs. His leukocyte count is 9,000/mm3 with 25% eosinophils. Further evaluation is most likely to show which of the following findings?

Rhabditiform larvae on stool microscopy

Entamoeba histolytica antibodies on stool immunoassay

Branching septate hyphae on KOH preparation

Oocysts on acid-fast stool stain

Giardia lamblia antibodies on stool immunoassay

Echinococci and other tissue helminths for USMLE Step 1: High-Yield Microbiology Lessons

Echinococcus - Cysts of Despair

Organism: Echinococcus granulosus (cystic), E. multilocularis (alveolar).
Transmission: Ingestion of eggs from canid (dog, fox) feces. Humans are accidental intermediate hosts; sheep are common intermediates.
Pathogenesis: Larvae form slow-growing hydatid cysts, primarily in the liver (~70%) and lungs.
- E. granulosus cysts contain "hydatid sand" (protoscolices).
- E. multilocularis is more invasive, mimicking malignancy.
Clinical: Often asymptomatic for years. Presents with RUQ pain, mass effect, or hepatomegaly.

⭐ Spillage of cyst fluid, either spontaneously or during surgery, can cause life-threatening anaphylactic shock and disseminate the infection.

Diagnosis:
- Imaging (US/CT/MRI) is key, showing characteristic cysts, often with "egg-shell" calcification or daughter cysts.
- Serology confirms exposure.
Treatment: Albendazole, PAIR (Puncture, Aspiration, Injection, Re-aspiration), or surgery.

CT scan of liver with multiple Echinococcus hydatid cysts

Taenia solium - Brain Invaders

Organism: Pork tapeworm (cestode).
Transmission & Disease:
- Taeniasis (Intestinal): Ingesting larvae (cysticerci) in undercooked pork.
- Cysticercosis (Tissue/Brain): Ingesting eggs from human feces (fecal-oral route).
Neurocysticercosis (NCC) Presentation:
- Seizures are the most common symptom.
- Headaches, focal neurological deficits, signs of increased intracranial pressure.
Diagnosis:
- Neuroimaging: CT or MRI reveals cystic lesions. Brain parenchyma may show a "Swiss cheese" appearance.
- Serology: Enzyme-linked immunoelectrotransfer blot (EITB) is highly specific.
Management:
- Antiparasitic therapy: Albendazole or Praziquantel.
- Corticosteroids are co-administered to reduce inflammation from dying cysts.

CT scan of brain with calcified neurocysticercosis lesions

⭐ Treatment with antiparasitic drugs can paradoxically worsen neurological symptoms due to an intense inflammatory reaction to dying cysticerci; this is why concurrent steroid administration is critical.

Other Tissue Worms - Migrating Menaces

Toxocara canis/cati (Visceral/Ocular Larva Migrans)
- Source: Ingestion of eggs from dog/cat feces.
- Visceral (VLM): Fever, hepatomegaly, marked eosinophilia in young children.
- Ocular (OLM): Unilateral vision loss, strabismus; mimics retinoblastoma.
- Tx: Albendazole.
Trichinella spiralis (Trichinosis)
- Source: Undercooked pork, bear, or wild game.
- Path: Larvae encyst in striated muscle.
- Sx: GI upset → fever, myalgia, splinter hemorrhages, periorbital edema.
- Labs: ↑ Eosinophils, ↑ Creatine Kinase (CK).
⭐ Classic Triad: Periorbital edema, myalgia, and eosinophilia.
Baylisascaris procyonis (Raccoon Roundworm)
- Source: Ingestion of eggs from raccoon feces.
- Path: Aggressive, often fatal, neural larva migrans.

Trichinella spiralis larvae encysted in muscle

High‑Yield Points - ⚡ Biggest Takeaways

Echinococcus granulosus (dog host) causes hydatid cysts; rupture risks anaphylaxis. Imaging shows eggshell calcification.

Taenia solium eggs cause neurocysticercosis (seizures). Larvae in undercooked pork cause intestinal taeniasis.

Trichinella spiralis (undercooked pork/bear) presents with myalgia, periorbital edema, and eosinophilia.

Toxocara canis (dog/cat feces) causes visceral and ocular larva migrans in children.

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