Final answer:
The HIV-positive patient's symptoms suggest Stevens-Johnson Syndrome (SJS) or Toxic Epidermal Necrolysis (TEN), likely triggered by allopurinol. This condition is a medical emergency and can be life-threatening in patients with compromised immune systems.
Step-by-step explanation:
The patient with HIV and a history of fever, targetoid skin lesions, and visual impairment, now presenting with lip/mouth ulceration, and a positive Nikolsky sign, is likely suffering from Stevens-Johnson Syndrome (SJS) or Toxic Epidermal Necrolysis (TEN). These severe skin reactions are often triggered by medications, and the recent start of allopurinol could be a causative factor. SJS/TEN are medical emergencies that require immediate attention, as they can lead to significant morbidity and mortality, especially in patients with a compromised immune system.