Final answer:
The most defective biochemical pathway in the patient is the pathway responsible for repairing DNA damage caused by ultraviolet (UV) radiation.
Step-by-step explanation:
The most defective biochemical pathway in the patient is the pathway responsible for repairing DNA damage caused by ultraviolet (UV) radiation.
One specific disorder that causes extreme sensitivity to sunlight and defective DNA repair is Xeroderma Pigmentosum (XP). In XP, the body is unable to repair DNA damage, specifically thymine dimers, caused by UV radiation. As a result, the skin develops vesicles and blisters that take weeks to heal.
UV radiation causes thymine dimers to form when two adjacent thymine bases in the DNA strand become covalently bonded. The normal DNA repair pathway is responsible for recognizing these dimers and removing them. However, in patients with XP, this repair pathway is defective, leading to persistent DNA damage and skin sensitivity to sunlight.