30th April 2024

A 10-year-old girl presents with a fever of 38.5°C, malaise, and an itchy rash that started on her trunk and spread to her face and limbs over the past two days.

A 10-year-old girl presents with a fever of 38.5°C, malaise, and an itchy rash that started on her trunk and spread to her face and limbs over the past two days. The rash initially appeared as red spots which have now progressed to fluid-filled blisters. There is no significant medical history, but her parents mention she was in contact with a schoolmate who had chickenpox two weeks ago.

Investigations:

Temperature: 38.5°C

Physical Examination: Diffuse vesicular rash at different stages of development, predominantly on the trunk and spreading to the face and limbs

Which investigation is most likely to confirm the diagnosis?