**Core Concept**
The patient's presentation is suggestive of a severe allergic reaction, specifically a type of cutaneous eruption that is common in pregnancy. This condition is characterized by the formation of blisters and intense pruritus, which worsens over time.

**Why the Correct Answer is Right**
The correct diagnosis in this case is Pemphigoid Gestationis (PG), a rare autoimmune disorder that occurs during pregnancy. It is believed to be caused by the transfer of paternal antigens to the maternal immune system, leading to an immune response against the fetal skin. The condition is characterized by the presence of autoantibodies against the basement membrane zone of the skin. The clinical presentation of PG includes extremely pruritic and urticarial papules and blisters that typically appear on the abdomen and trunk.

**Why Each Wrong Option is Incorrect**
* **Option A:** Polymorphic eruption of pregnancy (PEP) is a different condition that presents as a pruritic, polymorphic rash, but it does not typically cause blisters and is generally milder than PG.
* **Option B:** Pustular psoriasis is a type of psoriasis that presents with pustules, but it is not typically associated with pregnancy and does not usually cause blisters.
* **Option C:** Erythema multiforme is a skin condition characterized by target lesions, but it is not typically associated with pregnancy and does not usually cause blisters.
* **Option D:** Erythema nodosum is a skin condition characterized by painful nodules, but it is not typically associated with pregnancy and does not usually cause blisters.

**Clinical Pearl / High-Yield Fact**
Pemphigoid Gestationis is a rare but potentially severe condition that requires prompt diagnosis and treatment to prevent long-term skin damage and other complications. It is essential for obstetricians and dermatologists to be aware of this condition and to consider it in the differential diagnosis of any pregnant patient presenting with severe pruritus and a polymorphic rash.

**Correct Answer:** C. Erythema multiforme is a skin condition characterized by target lesions, but it is not typically associated with pregnancy and does not usually cause blisters.