Final answer:
Secondary keratinisation defects can be attributed to factors like malnutrition, viral infections, and diabetes, as well as genetic conditions such as xeroderma pigmentosa, which impairs DNA repair mechanisms and increases the risk of skin cancer.
Step-by-step explanation:
The causes of secondary keratinisation defects can include a variety of factors such as malnutrition, viral infection, diabetes, prolonged infections, chemical or radiation exposure, and particular genetic disorders like xeroderma pigmentosa. For instance, in the case of xeroderma pigmentosa, which is a genetic condition, affected individuals exhibit skin that is exceptionally sensitive to UV rays because of a defect in the nucleotide excision repair enzymes. This deficit leads to a failure to repair certain types of DNA damage, such as pyrimidine dimers, commonly those of thymine. Sun exposure in such individuals may result in skin lesions and a heightened possibility of developing skin cancers like basal cell carcinoma, squamous cell carcinoma, and melanoma due to the persistence of these thymine dimers that can distort the DNA double helix and affect DNA replication processes.