Major Diagnostic Categories. The Major Diagnostic Categories (MDC) are formed by dividing all possible principal diagnoses into 25 mutually exclusive diagnosis areaareas. 0 = Ungroupable 1 = Nervous System 2 = Eye 3 = Ear, Nose, Mouth, and Throat 4 = Respiratory System 5 = Circulatory System 6 = Digestive System 7 = Hepatobiliary System And and Pancreas 8 = Musculoskeletal System And and Connective Tissue 9 = Skin, Subcutaneous Tissue And , and Breast 10 = Endocrine, Nutritional And , and Metabolic System 11 = Kidney and Urinary Tract 12 = Male Reproductive System 13 = Female Reproductive System 14 = Pregnancy, Childbirth and Puerperium 15 = Newborn and Other Neonates (Prenatal Period) 16 = Blood and Blood Forming Organs and Immunological Disorder 17 = Myeloprolifeative DDs (Poorly Differentiated Neoplasm) | 