In the myogenic response,

Correct Answer: C

Rationale: The myogenic response (C) is smooth muscle contraction in response to stretch, regulating flow. A is skeletal muscle pump, B is cardiac, D is incorrect (endothelins constrict).

Correct Answer: C

Rationale: Calcium (Ca²⁺, C) is a positive inotrope, enhancing contractility by increasing sarcomere interaction.

Correct Answer: A

Rationale: Viagra and nitroglycerin (A) together cause severe hypotension; a 24-hour gap is required. Other options (B, C, D) are incorrect per manufacturer guidelines.

Correct Answer: D

Rationale: Sinus arrhythmia (D) is a normal variation where heart rate increases on inspiration (due to vagal inhibition) and decreases on expiration (vagal tone), common in dogs.

Correct Answer: D

Rationale: The spleen, while important for immune function and blood filtering, is not essential for life; people can survive without it (e.g., after splenectomy). The liver, pancreas, and heart are vital for metabolism, digestion, and circulation.