Hi, The dark and dry patches in groin and inframmamary areas would be most probably erythrasma. It is a bacterial infection caused by cornybacterium minnutissimum. It is an asymptomatic contagious disease. The patches might spread to other areas like axillae. It is a curable disease. You consult dermatologist to have tirm diagnosis. Antibiotic like azithromycine may be taken inproper dose for few weeks. And apply clindamycine gel twice a day. Dettol liquid may be used during bath instead of soap. DM,HIV or other immunocompromised conditions msy be ruled out. Improve immunity and hygiene to prevent relapse of the disease. I hope you got my answer. Thanks. Dr. Ilyas Patel MD