Pastia's lines are:
Streptococcal scarlet fever:
Group A (or occasionally groups C and G) streptococci causing pharyngitis, tonsillitis or other infection may lead to scarlet fever, if the infecting strain produces a streptococcal pyrogenic exotoxin. Scarlet fever is most common in school-age children, but can also occur in young adults who have contact with young children. A diffuse erythematous rash occurs, which blanches on pressure, classically with circumoral pallor. The tongue, initially coated, becomes red and swollen ('strawberry tongue'). The disease lasts about 7 days, the rash disappearing in 7-10 days, followed by a fine desquamation. Residual petechial lesions in the antecubital fossa may be seen ('Pastia's sign’). 
Treatment involves intravenous benzyl penicillin or an oral penicillin plus symptomatic measures.