A 15-year-old female arrives at a community health clinic complaining of a widespread rash that developed suddenly this morning. She describes the rash as itchy but denies significant pain. Her medical history includes well-controlled asthma and seasonal allergies. Two weeks prior, she started taking ibuprofen for patellofemoral pain after joining her school's cross-country team, which coincided with her transfer to a larger public high school. She maintains a pescatarian diet and takes a daily multivitamin. Her immunizations are noted to be incomplete, last updated at age 7. On examination, her temperature is 37.80C, blood pressure is 118/78 mmHg, pulse 75/min, respirations 14/min, and oxygen saturation 98% on room air. Physical examination reveals diffuse, symmetrical erythematous macules and papules, some confluent, on the trunk and extremities, which blanch with pressure. There are no significant lymphadenopathy, conjunctival injection, or oral lesions. Pulmonary examination is unremarkable with clear breath sounds. Based on her presentation, what is the most probable diagnosis and the immediate cornerstone of her management?

A 19-year-old woman is brought to her primary care provider's office by her parents due to a noticeable decline in her academic performance and changes in behavior over the past few months. Her parents report she has been struggling with university coursework, exhibiting reduced concentration, difficulty recalling recent events, and occasional disorientation, particularly in the mornings. They mention she was diagnosed with measles at age 7 following a family decision to follow an alternative vaccine schedule. Since then, she has generally been healthy, with no chronic conditions. Her paternal grandmother had a neurodegenerative disorder with choreiform movements. On examination, her vital signs are stable: temperature 37.0°C, blood pressure 115/75 mmHg, pulse 78/min, respirations 14/min. Neurological examination reveals she is disoriented to time and place, with a Mini-Mental State Examination score of 18/30. There is no nuchal rigidity or focal weakness. Over the past week, her parents have observed subtle, irregular, jerking movements primarily affecting her right arm, especially when she is stressed. Magnetic resonance imaging (MRI) of the brain is obtained, demonstrating diffuse cerebral atrophy with white matter signal changes. Which of the following is the most likely diagnosis and the critical initial step in her management?

A 7-year-old girl presents to an urgent care clinic with a spreading rash. Her parents report that she developed a low-grade fever, cough, runny nose, and red eyes about 3 days ago, and these symptoms have progressively worsened. The rash, which started behind her ears, has now spread downwards to cover her trunk and extremities. The parents recall that she missed some of her routine childhood vaccinations due to personal preferences. On physical examination, the girl appears lethargic, with notable conjunctivitis and coryza. Examination of the buccal mucosa reveals small, bluish-white spots on an erythematous base. Her vital signs include a temperature of 39.20C, heart rate of 120 bpm, respiratory rate of 22/min, blood pressure of 95/60 mmHg, and oxygen saturation of 98% on room air. The rash is blanching, erythematous, and maculopapular, consistent with the appearance in Figure A. Considering the clinical presentation, what is the most likely diagnosis and the immediate management strategy?

A 4-year-old girl is brought to an outpatient clinic by her parents with concerns about a generalized rash that began on her face two days prior and has since spread to her trunk and extremities. Her parents recall she had a prodromal phase about five days ago, characterized by a low-grade fever, persistent cough, and conjunctivitis with noticeable eye discharge, which resolved spontaneously before the rash appeared. On examination, her temperature is 37.2°C, blood pressure is 92/60 mmHg, pulse is 110/min, and respirations are 20/min. Oxygen saturation is 99% on room air. Inspection of her oral mucosa reveals resolving tiny, whitish spots on a red base on the buccal surface. Her skin exhibits a confluent, erythematous maculopapular eruption. Considering the presenting symptoms and physical findings, what is the most likely diagnosis and the appropriate initial management strategy?

A 15-year-old female presents to an outpatient clinic with a 3-day history of high-grade fever, generalized malaise, and a progressive rash. Her mother states she developed a cough, coryza, and conjunctivitis a few days prior to the rash onset. The family recently returned from an international trip, and the patient has an incomplete immunization record. On examination, her temperature is 39.8°C, blood pressure is 90/60 mmHg, pulse is 110/min, respirations are 20/min, and oxygen saturation is 97% on room air. Physical examination reveals a blanching, erythematous maculopapular rash that began on her face and behind her ears, now spreading centrifugally to her trunk and extremities. Small, irregular, bright red spots with bluish-white centers are noted on her buccal mucosa. Lab results are pending. Considering the clinical presentation and recent history, what is the most likely diagnosis, and what is the immediate priority in her acute management to prevent severe outcomes?

A 2-year-old girl is brought to an urgent care clinic by her parents due to a worsening febrile illness and a new rash. Her parents report she developed a fever of 39.8°C three days ago, along with a persistent cough, runny nose, and red eyes. Yesterday, a rash appeared on her face and behind her ears, which has since spread downwards to her trunk and upper extremities. She has not received any childhood vaccinations. On examination, the child appears lethargic and irritable. Her conjunctivae are injected, and careful oral examination reveals small, irregular, bright red spots with bluish-white centers on her buccal mucosa. Palpation of the neck reveals tender cervical lymphadenopathy. Given her symptoms and the need for immediate intervention, what is the most appropriate initial management strategy, and what specific measure can prevent future spread of this condition?