Hypersensitivity Reaction: This is the most likely diagnosis given the presence of an allergic rash. The reaction could be due to various allergens such as insect bites, certain medications, or contact dermatitis. The body's immune response to the allergen leads to the release of histamine and other chemical mediators, causing the allergic symptoms.

Anaphylaxis: Although the patient's symptoms are not fully indicative of anaphylaxis (which typically includes widespread rash, swelling, difficulty breathing, and a drop in blood pressure), it's a consideration due to the allergic rash and potential for rapid progression.
Contact Dermatitis: A localized skin reaction that occurs after contact with an allergen or irritant, which could explain the rash on one arm.
Insect Bite or Sting Reaction: A common cause of localized allergic reactions, which could be the cause of the rash on the left arm.

Anaphylactic Shock: Despite the current blood pressure being elevated, anaphylactic shock is a life-threatening condition that requires immediate recognition and treatment. The presence of a rash and bradycardia (low heart rate) could be indicative of a severe allergic reaction affecting multiple systems.
Stevens-Johnson Syndrome/Toxic Epidermal Necrolysis (SJS/TEN): Although rare, these are severe skin conditions usually triggered by medications or infections. They can present with skin lesions and are medical emergencies.
Cardiac Issues: The combination of hypertension and bradycardia could suggest underlying cardiac problems, such as heart block or other conduction abnormalities, which need prompt evaluation.

Mastocytosis: A condition characterized by the accumulation of mast cells in one or more organs, which can lead to episodes of allergic symptoms, including rash, due to the release of histamine and other mediators.
Erythema Multiforme: An immune-mediated skin condition characterized by target lesions, which could be considered if the rash has a distinctive appearance, although it's less likely given the description.
Pheochromocytoma: A rare tumor of the adrenal gland that could cause episodic hypertension and potentially other symptoms like rash if associated with a systemic mast cell disorder or other conditions. However, this would be an unusual presentation.