For the past 3 years, a 24-year-old college student has suffered from chronic headaches, fatigue, shortness of breath, dizziness, ringing ears, and constipation. He is incensed when his primary physician recommends a psychiatric evaluation because no organic cause for his symptoms could be found. What is the most likely diagnosis?

A. Somatization disorder

B. Specific phobia

C. Dysthymia

D. Conversion disorder