A family pedigree reveals first- and second-generation female relatives with premature menopause and male relatives with a progressive neurodegenerative disorder starting by their sixth decade. There are more males than females exhibiting mental retardation from childhood by the fourth generation. Genetic analysis of affected persons reveals CGG repeat expansions in a gene encoding for a protein that binds mRNA transcripts in neurons and shuttles them to the synapses. An abnormality involving which of the following organs is most likely to be present in affected males?