Monozygotic ("identical") twins have 50% of getting schizophrenia if their twin has it. Dizygotic twins have ~ 20% chance though. This is often mentioned when Schizophrenia etiology is discussed (i.e., we don't know but concordance studies suggest a genetic link). T1DM has less of a genetic link than T2DM but it also has 50% concordance between monozygotic twins. This fact is mentioned in First Aid 2019, page 346... I got this NBME question wrong but it's content is in board sources.
The way I approached this problem was, schizophrenia doesn't have a clear pattern of inheritance. Among the option choices, only Type 1 DM doesn't have a clear inheritance pattern (eg: CF is auto recessive, Fragile X is XD, and so on).
If anyone wouldn’t mind: How am I supposed to know that T1DM has a similar pedigree to schizophrenia? Teach me how to think, ples.
The way I approached this question was by thinking that schizophrenia has a multifactorial inheritance pattern(lots of enviroment influence). So I looked for a disease that also has a multifactorial inheritance pattern, thought of DM1 because it is an autoimmune disease and most of them have a multifactorial inheritance(ie you may have DR3 or DR4, but you need the enviromental influence to actually attack your own pancreas)