In a clinical study of human monozygotic (identical) twins who were separated at birth and raised in completely different geographical and socio-economic environments, researchers measured several physiological and biochemical characteristics. Which of the following traits would be expected to show 100% concordance (showing identical phenotypes in both twins) regardless of their different environments?
Serum immunoglobulin G (IgG) antibody profile against specific viral pathogens
Red blood cell rhesus (RhD) antigen status
Resting systolic blood pressure
Clinical severity of abdominal symptoms following lactose ingestion