Contact Dermatitis: Given the patient's recent hospitalization and exposure to various medical products, contact dermatitis is a plausible cause for the skin rash, especially if the rash is localized to the pelvic region where medical devices or products might have been applied.

Drug Rash (Pembrolizumab or Enfortumab): Both pembrolizumab and enfortumab are known to cause skin reactions as part of their side effect profile. The rash could be an adverse reaction to one of these medications.
Intertrigo: This is a type of skin inflammation caused by skin-on-skin friction, which could be exacerbated in the pelvic region, especially if the patient has been less mobile during hospitalization.
Candidal Intertrigo: Given the warm, moist environment of the pelvic region, a fungal infection such as candidal intertrigo could develop, especially if the patient's immune system is compromised due to cancer treatment.

Stevens-Johnson Syndrome (SJS) or Toxic Epidermal Necrolysis (TEN): Although less common, these severe skin conditions can be life-threatening and are associated with medication use, including some cancer therapies. The presence of blisters and the patient's recent medication regimen make this a critical diagnosis not to miss.
Invasive Fungal Infection: In immunocompromised patients, such as those on pembrolizumab, the risk of invasive fungal infections is increased. A skin rash could be the initial presentation of a more serious underlying infection.

Paraneoplastic Pemphigus: A rare autoimmune disorder associated with certain types of cancer, characterized by blistering of the skin and mucous membranes. Although rare, it's a consideration in a patient with stage 4 cancer.
Graft-Versus-Host Disease (GVHD): If the patient has undergone any form of bone marrow transplant or has received blood products, GVHD could be a rare but possible cause of the skin rash, especially if the rash is part of a broader systemic reaction.