Pathophysiological cascades in canine Gastric Dilatation-Volvulus (GDV):

• Mechanical compression of the caudal vena cava and portal vein: Massive gastric distension compresses intra-abdominal veins, leading directly to Reduced venous return (C) and Portal hypertension (D), severely diminishing cardiac output and precipitating obstructive/distributive shock.
• Diaphragmatic compromise: Upward cranial displacement of the diaphragm restricts tidal lung expansion, producing alveolar hypoventilation and carbon dioxide retention, which causes Respiratory acidosis (B).
• Acid-base status: Tissue hypoperfusion and anaerobic glycolysis induce lactic acidosis (Metabolic acidosis, NOT metabolic alkalosis; A is false).

Auscultatory and hemodynamic hallmarks of Patent Ductus Arteriosus (PDA):

• Continuous machinery murmur: Failure of the physiological closure of the fetal ductus arteriosus leaves a left-to-right shunt between the high-pressure descending aorta and the lower-pressure pulmonary artery. Because aortic pressure exceeds pulmonary arterial pressure throughout both systole and diastole, continuous turbulent blood flow generates a classic 'continuous machinery murmur', which peaks in late systole and is loudest over the left dorsal heart base (third or fourth intercostal space).
• Hyperkinetic ('waterhammer' / bounding) femoral pulse: The continuous diastolic runoff of blood from the aorta into the pulmonary artery causes low diastolic arterial blood pressure while systolic pressure remains preserved or elevated, resulting in a markedly wide pulse pressure that palpates as a sharp, hyperkinetic/bounding femoral pulse.
  
  Option analysis:
• (Patent ductus arteriosus): Correct. Pathognomonic triad of left cranial continuous murmur and hyperkinetic bounding pulse.
• Diastolic decrescendo murmur, not continuous throughout both systole and diastole.
• Produces myocardial failure and systolic murmurs of mitral regurgitation.
• Produces esophageal constriction and regurgitation without a continuous murmur.

Gastric disorders and their cardinal clinical signs:

• Acute gastritis: Characterized by a sudden, acute onset of vomiting -> (A) matches (IV).
• Ulceration or erosion: Disruption of mucosal barrier with bleeding results in 'coffee-ground' vomitus or fresh blood (hematemesis) -> (B) matches (I).
• Gastric dilatation/volvulus (GDV): Classic triad of unproductive retching, progressive abdominal distension, and hypovolemic shock with tachycardia -> (C) matches (II).
• Chronic gastritis: Vomiting persisting for more than $1\text{--}2\text{ weeks}$, often containing partially digested food or bile -> (D) matches (III).
  
  Option analysis:
• Acute gastritis -> (IV).
• Ulceration -> (I).
• GDV -> (II).
• Chronic gastritis -> (III). Matches Option 3.