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 38-year-old woman comes to the physician because of a 4-day history of swelling and pain in her left knee. She has had similar episodes of swollen joints over the past 3 weeks. Two months ago, she had a rash on her upper back that subsided after a few days. She lives in Pennsylvania and works as a forest ranger. Her temperature is 37.8°C (100°F). Physical examination shows a tender and warm left knee. Arthrocentesis of the knee joint yields cloudy fluid with a leukocyte count of 65,000/mm3 and 80% neutrophils. A Gram stain of synovial fluid does not show any organisms. Which of the following is the most likely cause of this patient's condition?

Infection with spiral-shaped bacteria

Production of autoantibodies against Fc portion of IgG

Wearing down of articular cartilage

Postinfectious activation of innate lymphoid cells of the gut

Infection with round bacteria in clusters

A 2-year-old boy presents for a routine checkup. The patient’s mother says that he has been ‘under the weather’ for the past few days. She did not measure his temperature at home but states that he has felt warm. She denies any episodes of diarrhea or vomiting. No significant past medical history or current medications. The patient attends daycare. He is due for a hepatitis A vaccine. The patient was born at term with no prenatal or perinatal complications. The vital signs include: temperature 37.8°C (100.1°F), blood pressure 112/62 mm Hg, pulse 80/min, respiratory rate 18/min, and oxygen saturation 99% on room air. The patient is alert and responsive. The physical exam is unremarkable. Which of the following is the most appropriate next step in the management of this patient?

Administer the hepatitis A vaccine

Strep rapid antigen detection test

Delay the hepatitis A immunization until next visit

A 4-year-old boy is brought to the clinic by his mother with fever and a rash. The patient’s mother says his symptoms started 1 week ago with the acute onset of fever and a runny nose, which resolved over the next 3 days. Then, 4 days later, she noted a rash on his face, which, after a day, spread to his neck, torso, and extremities. The patient denies any pruritus or pain associated with the rash. No recent history of sore throat, chills, or upper respiratory infection. The patient has no significant past medical history and takes no medications. The vital signs include: temperature 37.2°C (99.9°F) and pulse 88/min. On physical examination, there is a maculopapular rash on his face, torso, and extremities, which spares the palms and soles. The appearance of the rash is shown in the exhibit (see image below). Which of the following would most likely confirm the diagnosis in this patient?

ELISA for parvovirus B-19 IgM and IgG antibodies

Assay for IgM and IgG against measles virus

Serology for human herpesvirus-6 IgM antibodies

ELISA for IgM antibodies against Rubella virus

Five days after being admitted to the hospital for a scald wound, a 2-year-old boy is found to have a temperature of 40.2°C (104.4°F). He does not have difficulty breathing, cough, or painful urination. He initially presented one hour after spilling a pot of boiling water on his torso while his mother was cooking dinner. He was admitted for fluid resuscitation, nutritional support, pain management, and wound care, and he was progressing well until today. He has no other medical conditions. Other than analgesia during this hospital stay, he does not take any medications. He appears uncomfortable but not in acute distress. His pulse is 150/min, respirations are 41/min, and blood pressure is 90/50 mm Hg. Pulse oximetry on room air shows an oxygen saturation of 99%. Examination shows uneven, asymmetrical scalding covering his anterior torso in arrow-like patterns with surrounding erythema and purulent discharge. The remainder of the examination shows no abnormalities. His hemoglobin is 13.4 g/dL, platelet count is 200,000/mm3, and leukocyte count is 13,900/mm3. Which of the following is the most appropriate initial pharmacological treatment for this patient?

Amoxicillin/clavulanic acid and ceftriaxone

Ampicillin/sulbactam and daptomycin

Piperacillin/tazobactam and cefepime

A 5-year-old boy presents to your office with his mother. The boy has been complaining of a sore throat and headache for the past 2 days. His mother states that he had a fever of 39.3°C (102.7°F) and had difficulty eating. On examination, the patient has cervical lymphadenopathy and erythematous tonsils with exudates. A streptococcal rapid antigen detection test is negative. Which of the following is the most likely causative agent?

An enveloped, double-stranded DNA virus

A gram-negative, pleomorphic, obligate intracellular bacteria

An enveloped, single-stranded, negative sense RNA virus

A gram-positive, beta-hemolytic cocci in chains

A naked, double-stranded DNA virus

A 12-year-old boy is brought in by his mother to the emergency department. He has had abdominal pain, fever, nausea, vomiting, and loss of appetite since yesterday. At first, the mother believed it was just a "stomach flu," but she is growing concerned about his progressive decline. Vitals include: T 102.3 F, HR 110 bpm, BP 120/89 mmHg, RR 16, O2 Sat 100%. Abdominal exam is notable for pain over the right lower quadrant. What is the next best step in management in addition to IV hydration and analgesia?

Right lower quadrant ultrasound

Upright and supine abdominal radiographs

Abdominal MRI with gadolinium contrast

Abdominal CT scan with IV contrast

Abdominal CT scan with IV and PO contrast

Fever of unknown origin approach for USMLE Step 1: High-Yield Pediatrics Lessons

FUO Definition - The Heat is On

Classic Criteria (Petersdorf & Beeson):
- Temperature > 38.3°C (101°F) on multiple occasions.
- Duration of illness ≥ 3 weeks.
- No established diagnosis after 1 week of inpatient investigation.
Pediatric Modification:
- Fever duration ≥ 8 days with no clear cause after initial outpatient/inpatient evaluation.

⭐ In children, most FUOs are unusual presentations of common diseases (e.g., UTI, EBV), not rare esoteric conditions.

Etiology - The Usual Suspects

📌 Mnemonic: "I-N-C"

Infections (~50%): Most common cause.
- Bacterial: Tuberculosis (esp. extrapulmonary), Typhoid fever, UTI, Occult abscess.
- Viral: Epstein-Barr Virus (EBV), Cytomegalovirus (CMV).
- Parasitic: Malaria.
Neoplasms (~10%):
- Leukemia (ALL).
- Lymphoma (Hodgkin & Non-Hodgkin).
Collagen Vascular / Rheumatic (~10-15%):
- Systemic-onset Juvenile Idiopathic Arthritis (SoJIA / Still's Disease).
- Systemic Lupus Erythematosus (SLE).
- Kawasaki Disease (especially in infants).

⭐ Exam Favourite: Systemic-onset JIA (Still's Disease) is a classic non-infectious cause of FUO, often presenting with a quotidian (daily) fever spike, evanescent salmon-pink rash, and arthritis.

Diagnostic Workup - The Fever Detective

Goal: Systematically narrow down differentials without excessive, low-yield testing.
Approach: Phased, moving from non-invasive to invasive based on clinical clues.

⭐ In children, while infections are the most common cause of FUO, non-infectious inflammatory diseases (like Juvenile Idiopathic Arthritis) and malignancies become increasingly likely as the fever persists beyond 2-3 weeks.

Age-Specific Clues - Little Patients, Different Clues

Neonates (<28 days):
- Etiology: Perinatal infections (TORCH, Listeria), Group B Strep, E. coli.
- Signs: Non-specific; lethargy, poor feeding, irritability, hypothermia.
Infants (1-3 months):
- Etiology: Late-onset GBS, UTI (E. coli), occult bacteremia.
- Consider Herpes Simplex Virus (HSV) if vesicles or seizures present.
Young Children (3-36 months):
- Etiology: UTI, pneumonia, Kawasaki disease, Roseola (HHV-6).
Older Children (>3 years):
- Etiology: Localized infections (pharyngitis, sinusitis), EBV, CMV, Mycoplasma.
- Non-infectious: Juvenile Idiopathic Arthritis (JIA), malignancy (leukemia).

⭐ In infants < 3 months with FUO, Urinary Tract Infection (UTI) is a common and serious bacterial cause that must be ruled out.

High‑Yield Points - ⚡ Biggest Takeaways

Infections are the #1 cause of pediatric FUO, followed by collagen vascular diseases.

Defined as fever >38.3°C for >8 days with no source after initial workup.

Juvenile Idiopathic Arthritis (JIA) is the most common rheumatological cause.

Key infections: Bartonella henselae (Cat Scratch), EBV, CMV, and tuberculosis.

In neonates and young infants, always rule out sepsis and Herpes Simplex Virus (HSV).

Leukemia and lymphoma are the most critical malignant causes to exclude.

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