Welcome to XXXXXXX forum.
Svere itching in pubic region along with skin discoloration may be due to fungal infection
which is called as "Tinea
This infection is very common in males and spread by contaminated hands.
Although you have history of scalp psoriasis
but possibility appears to be negligible.
You should consult with XXXXXXX medicine specialist/ Dermatologist
and should go for thorough check up.
You should go for KOH mount preparation for detection of fungal hyphae.
You should also take anti-fungal medicines under strict guideance of your treating doctor.
You should avoid soap application at infected site.
Avoid tight and rough clothes.
Wear soft cloth undergarments.
You should also apply anti-fungal powder at infected site.
Hope for speedy recovery.
Let me know if I can address any more concerns.
Dr. Mayank Bhargava