Fanconi Anemia — Thumb Abnormalities

A 6-year-old boy presents with a 1-year history of easy bruisability, recurrent epistaxis, progressive pallor, and recurrent upper respiratory infections. On examination, he is short (<3rd centile), microcephalic, with generalized hyperpigmentation and café-au-lait macules. He has bilateral absent thumbs with radial ray defects (absent radius), triangular facies, microphthalmia, low-set ears, and genitourinary malformations. Laboratory studies reveal pancytopenia with macrocytic anemia, thrombocytopenia, and neutropenia; bone marrow is hypocellular. The structure marked **C** in the diagram represents the characteristic skeletal and hematologic abnormalities of this condition. Which of the following is the gold-standard diagnostic test that would confirm the diagnosis?