A previously healthy 6-year-old boy is brought to the physician because of a 3-day history of progressive rash. The rash started on his face and now involves the entire body. For the past week, he has had a cough and a runny nose. He is visiting from the Philippines with his family. He is in first grade and spends his afternoons at an after-school child care program. Immunization records are not available. His temperature is 39.5°C (103°F), pulse is 115/min, and blood pressure is 105/66 mm Hg. Examination shows generalized lymphadenopathy. There is an erythematous maculopapular, blanching, and partially confluent exanthem on his entire body. The remainder of the examination shows no abnormalities. Which of the following is most likely to confirm the diagnosis?

Measles-specific IgM antibodies

Rapid antigen detection testing

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

A 15-year-old Caucasian male is brought to his pediatrician by his parents, who note the development of a tremor in their child. Urine and serum analysis reveal elevated levels of copper. Which of the following clinical manifestations would the physician most expect to see in this patient?

Hepatocytes that stain with Prussian blue

A 72-year-old man is brought to your office by his daughter due to concern over recent behavioral changes. Over the last several months he has had increasing difficulty with remembering recent events. She mentions that he is embarrassed due to a new inability to control urination. His medical history is significant for hypertension and insomnia. His medications include alprazolam and hydrochlorothiazide. On physical exam, he is oriented to time and place and thinks his daughter is exaggerating; however, when asked to recall 3 items, the patient refuses to continue the mental status exam. He has 5/5 strength bilaterally. He walks in short strides by sliding his feet across the floor. Which of the following would you expect to see in this patient?

Distortion of corona radiata fibers

Depigmentation of the substantia nigra pars compacta

Convex hemorrhage that does not cross suture lines

Atrophy of the caudate and putamen

Atrophy of the subthalamic nucleus

A 62-year-old woman comes to the physician because of worsening mental status over the past month. Her husband reports that she was initially experiencing lapses in memory but has recently started having difficulties performing activities of daily living. She appears withdrawn and avoids eye contact. Examination shows diffuse involuntary muscle jerking that can be provoked by loud noises. A cerebrospinal fluid analysis shows elevated concentration of 14-3-3 protein. Four months later, the patient dies. Pathologic examination of the brain on autopsy is most likely to show which of the following findings?

Spongiform vacuolation of the cortex

Degeneration of the substantia nigra pars compacta

Focal inflammatory demyelination and gliosis

Deposits of amyloid beta peptides

Marked atrophy of caudate and putamen

Emerging infectious diseases in children for USMLE Step 2 CK: High-Yield Pediatrics Lessons

Emerging Infections - The New Challengers

COVID-19 (SARS-CoV-2):
- Key concern: MIS-C (Multisystem Inflammatory Syndrome), 2-6 weeks post-infection.
- Features: Kawasaki-like presentation, myocarditis, shock.
Zika Virus:
- Aedes vector.
- Congenital Syndrome: Severe microcephaly, intracranial calcifications.
Nipah Virus:
- Reservoir: Fruit bats. High mortality (>70%).
- Presents as acute encephalitis.
Scrub Typhus:
- Vector: Trombiculid mite (chigger).
- Classic triad: Fever, rash, eschar.

⭐ A painless eschar with a black necrotic center at the chigger bite site is pathognomonic for Scrub Typhus.

Scrub typhus rash and eschar

Viral Threats - Fevers & Fear

Nipah Virus (NiV)
- Reservoir: Fruit bats (Pteropus spp.); pigs are intermediate hosts.
- Presents as fever, headache, myalgia → rapidly progressing to encephalitis, seizures, coma.
- Features: Segmental myoclonus, areflexia, respiratory distress.
- Dx: Real-time PCR (RT-PCR) on throat swab, CSF, urine, blood.
- Rx: Supportive care; Ribavirin efficacy is debated.
Chandipura Virus (CHPV)
- Vector: Sandfly (Phlebotomus spp.).
- Affects children <15 years; presents as Acute Encephalitis Syndrome (AES).
- Sudden onset high fever, vomiting, altered sensorium, seizures.

⭐ Chandipura Virus has a very high case fatality rate, often >75%, with death occurring within 24-48 hours of onset.

Nipah virus zoonotic transmission cycle

Bacterial & Rickettsial - Hidden Dangers

Kingella kingae: A leading cause of septic arthritis & osteomyelitis in children <4 years. Often culture-negative; diagnosis relies on NAAT/PCR of synovial fluid.
Capnocytophaga canimorsus: Fulminant sepsis following dog bites, particularly dangerous for asplenic individuals.
Scrub Typhus (Orientia tsutsugamushi): Transmitted by chiggers (larval mites). Presents with high fever, headache, and myalgia. A painless eschar with a black necrotic center is pathognomonic.
Indian Tick Typhus (Rickettsia conorii): Characterized by fever, maculopapular rash starting on palms/soles, and an inoculation eschar (tache noire).

⭐ In suspected rickettsial diseases (like Scrub or Tick Typhus), treatment with Doxycycline should be initiated empirically without waiting for lab confirmation. It is the drug of choice across all age groups.

Syndromic Approach - Clinical Detective Work

Prioritizes early, life-saving interventions based on recognizable clinical patterns (syndromes) before definitive diagnosis is available.
Crucial for outbreaks (e.g., COVID-19, Nipah) where rapid response is key.
Common Pediatric Syndromes & Differentials:
- Fever + Rash: Measles, Dengue, Scrub Typhus, Chikungunya.
- Acute Encephalitis Syndrome (AES): Japanese Encephalitis (JE), Herpes Simplex (HSV), Enteroviruses.
- Hemorrhagic Fever: Dengue, Kyasanur Forest Disease (KFD).
- Acute Respiratory Distress: Influenza (H1N1), COVID-19, Hantavirus.

⭐ A triad of fever, rash, and thrombocytopenia strongly suggests Dengue, but always consider other possibilities like scrub typhus or meningococcemia in the differential diagnosis.

High‑Yield Points - ⚡ Biggest Takeaways

Key COVID-19 complication is Multisystem Inflammatory Syndrome in Children (MIS-C), presenting weeks post-infection.

Zika virus infection during pregnancy is strongly linked to congenital microcephaly in the fetus.

Dengue warning signs (abdominal pain, persistent vomiting) precede the critical phase of plasma leakage.

Chikungunya is characterized by abrupt high fever and severe, often incapacitating, polyarthralgia.

Scrub typhus presents with fever, rash, and a pathognomonic eschar at the chigger bite site.

Nipah virus is a high-mortality zoonosis causing severe respiratory illness and encephalitis.

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