Q. A one and a half year male child  develops high grade fevers of upto 39° F. The fever persists for 3 days.
On the fourth day, the fever resolves rather abruptly. A rash appears within 24 hours of fever resolution. The rash is rose colored, discrete,small, raised pinkish lesions on the trunk that spread to neck and face. The rash fades  after 2 days. The most likely organism involved is:

a}Parvovirus
b}Human herpes virus
c}Measles virus
d}Rubella virus
e}Rickettsia

Answer: b
Explanation:
Roseola infantum is caused  by human herpes virus. It occurs in children  less than 3 years of age. After a short prodrome, clinical illness is heralded by high  fever which resolves with a crisis {abruptly} or lysis{24-36 hours}. Rash appears 12-24 hours after fever resolution.

In measles, fever persists when rash appears. Koplik spots are pathognomonic of measles.

In rubella (German measles) the fever and rash are less striking. There is posterior auricular lymphadenopathy.

In rickettsial infections the face is spared & the rash is hemorrhagic.

Parvovirus causes erythema infectiosum (Fifth disease) with a distinctive rash of slapped cheek appearance.