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.
The erythematous streaks seen in staphylococcal infection
Transverse red streaks in skin folds seen in scarlet fever
White striae seen in lichen planus
The erythematous rash in syphilis