A 10-year-old boy is brought to the emergency department by his parents because of a dull persistent headache beginning that morning. He has nausea and has vomited twice. During the past four days, the patient has had left-sided ear pain and fever, but his parents did not seek medical attention. He is from Thailand and is visiting his relatives in the United States for the summer. There is no personal or family history of serious illness. He is at the 45th percentile for height and 40th percentile for weight. He appears irritable. His temperature is 38.5°C (101.3°F), pulse is 110/min, and blood pressure is 98/58 mm Hg. The pupils are equal and reactive to light. Lateral gaze of the left eye is limited. The left tympanic membrane is erythematous with purulent discharge. There is no nuchal rigidity. Which of the following is the most appropriate next step in management?

Intravenous ceftriaxone and clindamycin therapy

Intravenous cefazolin and metronidazole therapy

A 15-year-old boy is brought to the Emergency department by ambulance from school. He started the day with some body aches and joint pain but then had several episodes of vomiting and started complaining of a terrible headache. The school nurse called for emergency services. The boy was born at 39 weeks gestation via spontaneous vaginal delivery. He is up to date on all vaccines and is meeting all developmental milestones. Past medical history is noncontributory. He is a good student and enjoys sports. At the hospital, his blood pressure is 120/80 mm Hg, heart rate is 105/min, respiratory rate is 21/min, and his temperature is 38.9°C (102.0°F). On physical exam, he appears drowsy with neck stiffness and sensitivity to light. Kernig’s sign is positive. An ophthalmic exam is performed followed by a lumbar puncture. An aliquot of cerebrospinal fluid is sent to microbiology. A gram stain shows gram-negative diplococci. A smear is prepared on blood agar and grows round, smooth, convex colonies with clearly defined edges. Which of the following would identify the described pathogen?

Oxidase-positive and ferments glucose and maltose

Oxidase-positive test and ferments glucose only

Catalase-negative and oxidase-positive

No growth on Thayer-Martin medium

A 34-year-old woman presents with confusion, drowsiness, and headache. The patient’s husband says her symptoms began 2 days ago and have progressively worsened with an acute deterioration of her mental status 2 hours ago. The patient describes the headaches as severe, localized to the frontal and periorbital regions, and worse in the morning. Review of symptoms is significant for a mild, low-grade fever, fatigue, and nausea for the past week. Past medical history is significant for HIV infection for which she is not currently receiving therapy. Her CD4+ T cell count last month was 250/mm3. The blood pressure is 140/85 mm Hg, the pulse rate is 90/min, and the temperature is 37.7°C (100.0°F). On physical examination, the patient is conscious but drowsy. Papilledema is present. No pain is elicited with extension of the leg at the knee joint. The remainder of the physical examination is negative. Laboratory findings, including panculture, are ordered. A noncontrast CT scan of the head is negative and is followed by a lumbar puncture. CSF analysis is significant for: Opening pressure 250 mm H2O (70-180 mm H2O) Glucose 30 mg/dL (40-70 mg/dL) Protein 100 mg/dL (<40 mg/dL) Cell count 20/mm3 (0-5/mm3) Which of the following additional findings would most likely be found in this patient?

CSF India ink stain shows encapsulated yeast cells

Gram-positive diplococci are present on microscopy

CSF shows a positive acid-fast bacillus stain

Multiple ring-enhancing lesions are seen on a CT scan

CSF shows gram negative diplococci

A 42-year-old man is brought to the physician 25 minutes after an episode of violent jerky movements of his hands and legs that lasted for 5 minutes. After the episode, he had difficulty conversing. For the past 10 days, he has had a left-sided headache and nausea. Apart from a history of recurrent ear infections treated with antibiotics, he reports no other personal or family history of serious illness. He works as an assistant at a veterinarian clinic. He appears ill and is oriented to place and person only. His temperature is 37.8°C (100°F), pulse is 102/min, and blood pressure 112/78 mm Hg. Examination shows bilateral optic disc swelling. There is no lymphadenopathy. Muscle strength and tone is normal in all extremities. Deep tendon reflexes are 2+ bilaterally. Plantar reflex shows a flexor response bilaterally. Laboratory studies show a CD4 count within the reference range. An MRI of the brain is shown. Intravenous mannitol and levetiracetam are administered. Which of the following is the most appropriate next step in management?

Aspiration and surgical drainage

Pyrimethamine and sulfadiazine therapy

Ciprofloxacin and metronidazole therapy

Metronidazole and cefotaxime therapy

A 32-year-old woman presents with a 3-month history of intermittent blurred vision and problems walking. The patient states that she often feels “pins and needles” in her legs that cause her problems when she’s walking. The patient is afebrile, and her vital signs are within normal limits. An autoimmune disorder is suspected. Which of the following findings would most likely be present in this patient?

Damaged myelin sheath and myelin-producing cells

Destruction of blood-brain barrier

Degeneration of anterior horn cells

Decreased cerebrospinal fluid due to destruction of cells

A 3-year-old boy is brought to the physician for the evaluation of recurrent skin lesions. The episodes of lesions started at the age of 3 months. He has also had several episodes of respiratory tract infections, enlarged lymph nodes, and recurrent fevers since birth. The boy attends daycare. The patient's immunizations are up-to-date. He is at the 5th percentile for length and 10th percentile for weight. He appears ill. Temperature is 38°C (100.4°F). Examination shows several raised, erythematous lesions of different sizes over the face, neck, groin, and extremities; some are purulent. Bilateral cervical and axillary lymphadenopathy are present. What is the most likely underlying mechanism of this patient's symptoms?

Defective cytoplasmic tyrosine kinase

Impaired signaling to actin cytoskeleton reorganization

Defective neutrophil chemotaxis

Impaired repair of double-strand DNA breaks

Central nervous system infections for USMLE Step 3: High-Yield Internal Medicine Lessons

Meningitis - Braincoat on Fire

Classic Triad: Fever, nuchal rigidity, altered mental status (AMS).
Physical Signs: Kernig's (pain on knee extension), Brudzinski's (neck flexion → hip/knee flexion).
Etiology by Age:
- Neonate (<1 mo): GBS, E. coli, Listeria.
- Child/Adult: S. pneumoniae, N. meningitidis.
- >50 / Immunocompromised: Add Listeria coverage (Ampicillin).

CSF Analysis in Meningitis: Bacterial, Viral, Fungal

CSF Findings:
- Bacterial: ↑↑WBC (Neutrophils), ↓Glucose (<40 mg/dL), ↑Protein, ↑Opening Pressure.
- Viral: ↑WBC (Lymphocytes), Normal Glucose, Normal/↑Protein.
- Fungal/TB: ↑WBC (Lymphocytes), ↓Glucose, ↑Protein.

⭐ Give dexamethasone 15-20 min before or with the first dose of empiric antibiotics to reduce neurological sequelae (esp. from S. pneumoniae).

Encephalitis - Gray Matter Mayhem

Definition: Inflammation of the brain parenchyma, primarily affecting gray matter. Differentiated from meningitis by the presence of abnormal brain function (altered mental status, motor/sensory deficits, seizures).
Etiology:
- Most common sporadic: Herpes Simplex Virus-1 (HSV-1).
- Epidemics: Arboviruses (e.g., West Nile Virus, St. Louis encephalitis).
Diagnosis:
- MRI: Essential for localization.
- LP & CSF analysis: PCR for viral DNA is the gold standard.
Treatment:
- Start IV Acyclovir empirically for any suspected case to cover for devastating HSV encephalitis.

⭐ High-Yield: HSV-1 encephalitis shows a classic predilection for the temporal lobes, leading to focal deficits like aphasia, seizures, or personality changes.

MRI brain scan: HSV encephalitis vs. normal

Brain Abscess - Pus in the Processor

Etiology: Direct spread from sinusitis, otitis media, dental infection; hematogenous from endocarditis or lung infections.
Pathogens: Streptococcus (esp. S. viridans, S. intermedius), Staphylococcus aureus, anaerobes (Bacteroides).
Presentation: Headache is the most common symptom. The classic triad (headache, fever, focal deficits) is rare. Seizures are frequent.
Diagnosis: Ring-enhancing lesion on CT/MRI with contrast. Restricted diffusion on DWI is key.
Management: Aspiration/drainage + prolonged antibiotics (4-8 weeks) like ceftriaxone + metronidazole.

Brain MRI: Ring-enhancing lesions differential diagnosis

⭐ The central core of a bacterial abscess shows restricted diffusion on DWI MRI, helping to distinguish it from a necrotic tumor which typically does not.

Spinal Epidural Abscess - Spine's Pus Pocket

Patho: Pus collection in the epidural space, compressing the spinal cord.
Risks: IV drug use, spinal procedures (e.g., epidural anesthesia), immunosuppression (diabetes), distant infection (e.g., skin).
Classic Triad: 1. Fever, 2. Focal back pain, 3. Neurologic deficits.
- 📌 F-B-N Triad: Fever, Back pain, Neurologic deficits.
Dx: MRI with gadolinium is the gold standard test.
Tx: Urgent surgical decompression (laminectomy) + prolonged IV antibiotics (e.g., Vancomycin + Ceftriaxone).

⭐ Neurologic deficits can progress rapidly from radiculopathy to paralysis within hours to days; urgent diagnosis is critical.

Spinal Epidural Abscess with Cord Compression

High‑Yield Points - ⚡ Biggest Takeaways

CSF analysis is key: bacterial meningitis has ↑ protein, ↓ glucose, and neutrophils. Viral meningitis has normal glucose and lymphocytes.

Start empiric antibiotics (ceftriaxone, vancomycin) immediately after LP. Add ampicillin for extremes of age to cover Listeria.

HSV-1 is the most common cause of fatal sporadic encephalitis, affecting the temporal lobes. Treat urgently with IV acyclovir.

Cryptococcus neoformans causes meningitis in immunocompromised patients (CD4 < 100).

N. meningitidis is linked to a petechial rash and outbreaks in close quarters (dorms, barracks).

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