• Diagnostic Hallmark of Canine IMHA: Spherocytes are the hallmark erythrocyte morphological abnormality pathognomonic for Immune-Mediated Hemolytic Anemia (IMHA) in dogs.
• Mechanism of Spherocyte Formation: When circulating erythrocytes are opsonized with anti-erythrocyte autoantibodies (IgG) and/or complement ($C3b$), splenic and hepatic macrophages partially phagocytose portions of the cell membrane without lysing the whole cell. The damaged RBC reduces its surface-area-to-volume ratio, transforming into a small, rigid, densely staining spherical red blood cell that lacks central pallor.
• Contrasting Erythrocyte Morphologies:
• Acanthocytes: Irregularly spaced, blunt, spurlike projections seen in hemangiosarcoma and liver disorders.
• Schistocytes: Fragmented RBCs formed by mechanical shear stress in DIC.
• Echinocytes: Uniformly scalloped, crenated spikes associated with uremia or artifactual in vitro drying.

Correct option: (4) Spherocytes

• Mechanism of Fibrinoid Necrosis: Fibrinoid necrosis is a specialized form of tissue death typically involving the walls of blood vessels (vasculitis, malignant hypertension, Arthus reaction, polyarteritis nodosa).
• Depositional Components: Arises when severe immune-complex deposition (Type III hypersensitivity) triggers complement activation and neutrophilic infiltration, disrupting endothelial integrity. Circulating (A) Immunoglobulins, (B) Complement components ($C3$, $C1q$), and (C) Plasma proteins leak into the vessel wall alongside extravasated (D) Fibrin, polymerizing into an amorphous, bright pink, refractile smudgy deposit on H&E staining resembling fibrin.

Correct option: (3) (A), (B), (C) and (D).