• Ketogenesis During Prolonged Fasting:
• During starvation in fish, adipose and muscle tissues undergo lipolysis, flooding the liver with fatty acids undergoing $\beta$-oxidation to produce abundant Acetyl-CoA.
• Concurrently, hepatic oxaloacetate is heavily consumed by active gluconeogenesis to maintain circulating blood glucose levels.
• Because oxaloacetate is depleted, acetyl-CoA cannot condense with oxaloacetate to enter the citric acid cycle.
• Excess mitochondrial acetyl-CoA is diverted into ketogenesis, forming Acetoacetate via thiolase, HMG-CoA synthase, and HMG-CoA lyase (which is further reduced to $\beta$-hydroxybutyrate or decarboxylated to acetone).
• nalysis:
• (Acetoacetate): Correct.

Correct option: (3) Acetoacetate

• Biosynthesis of Eicosanoids:
• Arachidonic acid (ARA, 20:4n-6) is a 20-carbon polyunsaturated fatty acid liberated from membrane phospholipids by the action of phospholipase $A_2$ ($PLA_2$).
• It serves as the primary substrate for cyclooxygenase (COX-1 / COX-2), which catalyzes the cyclization and oxygenation of ARA into prostaglandin $G_2$ ($PGG_2$) and $PGH_2$, leading directly to the synthesis of prostaglandins (2-series $PGE_2, PGF_{2\alpha}$), prostacyclins, and thromboxanes.
• nalysis:
• (Arachidonic acid): Correct.

Correct option: (4) Arachidonic acid

• Overall Stoichiometry of the Citric Acid Cycle:
• For each acetyl moiety entering the cycle:

• Highlights:
• molecules of $CO_2$ are released.
• molecules of $NAD^+$ are reduced to $NADH$.
• molecule of $FAD$ is reduced to $FADH_2$.
• high-energy phosphate bond is generated via substrate-level phosphorylation ($GTP$).
• molecule of coenzyme A ($HS-CoA$) is regenerated.
• nalysis:
• Correct net stoichiometry equation.

Correct option: (3) Acetyl CoA + 3NAD+ + FAD + GDP + Pi + 2H2O -> 2CO2 + 3NADH + FADH2 + GTP + 3H+ + HS-CoA

• Physiological Effects of Insulin:
• Insulin is an anabolic peptide hormone synthesized and secreted by the $\beta$-cells of the pancreatic islets (Brockmann bodies in teleost fishes) in response to hyperglycemia and aminoacidemia.
• Induction of Glycogenesis:
• Insulin dephosphorylates and activates glycogen synthase while simultaneously inactivating glycogen phosphorylase, promoting the conversion of intracellular glucose into glycogen stores in liver and muscle.
• It suppresses counter-regulatory catabolic pathways: glycogenolysis and gluconeogenesis.
• nalysis:
• (Glycogenesis): Correct.

Correct option: (2) Glycogenesis