Stubborn dandruff may be caused by a fungus on your scalp that causes the skin cells to accumulate and become flaky. Scratching too much could also be the cause of a sore head. Use a medicated shampoo that treats the fungus and calms your scalp; additionally, wash your hair gently and frequently.

Psoriasis can be a bit challenging to treat but various treatments like medications, laser treatments, alternative treatments like homeopathy etc., avoiding triggers that worsen your psoriasis symptoms can prove useful. I suggest talking to your doctor for proper examination of your condition which will help in deciding which is the best treatment for your condition. 

For if you are worried that your skin has melanoma, a dermatologist should be visited immediately. Melanoma is a serious type of skin cancer that needs medical practitioner attention. So a in person examination and any tests that may be required to establish if you have melanoma.

According to your symptoms, a fungal infection or an irritation might be the reason behind it. The cracked foreskin could result from either the infection or the dryness. The pinkish skin around the anal area might be related. Hygiene is the first thing that must be done to make this area stay clean and dry. Antifungal cream or simple moisturizer may be required. Keep away from strong soaps and wear loose clothes. Drink plenty of liquids and eat a balanced diet to aid the natural healing process.

You might have a medical condition named acanthosis nigricans. It can sometimes cause the lower face to be darker than the rest. It's not dangerous but could be a sign of something more serious, like insulin resistance, that is happening inside your body. You can fix this by eating cleaner, being active, and maintaining skin health. Furthermore, consume a lot of water.