Seborrhoeic dermatitis

A 28-year-old man presents to the office because of flaky, itchy, white scales on the scalp. He is unable to recall its onset but states that it has been bothering him for “a while now.” He washes his scalp with mild shampoo once every two days and his face with cold water twice daily. Past medical history is noncontributory. Vitals are within normal limits. During the encounter, he repeatedly scratches his face over the area between the nose and the mouth. Physical examination shows greasy scaliness of the skin of the face and the area behind the ear as shown:  


 Reproduced from: Wikimedia Commons


Reproduced from: flickr.com


Which of the following is the most likely cause of this patient’s condition?