Okay, let's tackle this question about vitamin K-dependent proteins C and S. The question is asking which factors they inactivate. I remember that protein C and S are involved in anticoagulation, so they probably target clotting factors.

First, the core concept here is the anticoagulant pathway. Vitamin K is essential for the synthesis of certain clotting factors and anticoagulants. Protein C and protein S are part of the natural anticoagulant system. They work to prevent excessive blood clotting by inactivating specific factors.

Protein C is activated by thrombin when it binds to thrombomodulin. Once activated, along with protein S as a cofactor, they form a complex that inactivates clotting factors. The main factors they target are factors Va and VIIIa. These factors are part of the prothrombinase complex and the tenase complex, respectively. By inactivating them, protein C and S reduce thrombin generation, thus preventing excessive clot formation.

Now, looking at the options. The correct answer should be factors Va and VIIIa. Let me check the options again. Suppose the options are factors like V, VIII, IX, X, etc. If an option lists V and VIII, that's likely correct. The incorrect options might include other factors like IX or X, which are actually inactivated by other anticoagulants like antithrombin or protein C/S might not target them directly.

Wait, protein C/S mainly inactivate Va and VIIIa. So any option that includes these two is correct. If another option includes factors IX or X, those are incorrect because those are targets of other mechanisms. For example, antithrombin inactivates thrombin (IIa), IXa, Xa, XIa, XIIa. Factor V is inactivated by activated protein C, and factor VIIIa is also a target. So the correct answer must be factors Va and VIIIa.

Clinical pearl: Remember that protein C and S are part of the natural anticoagulant system, their deficiency can lead to thrombophilia. The inactivation of Va and VIIIa is key in preventing clot propagation. A helpful mnemonic is "C and S take out Va and VIIIa."

**Core Concept** Vitamin K-dependent proteins C and S are critical anticoagulants that regulate blood clotting by degrading activated clotting factors. Their activation via thrombin-thrombomodulin complexes initiates the protein C pathway, essential for preventing excessive thrombosis.

**Why the Correct Answer is Right** Activated protein C (APC), with protein S as a cofactor, inactivates **factor Va and VIIIa**. Factor Va is part of the prothrombinase complex, and VIIIa is a cofactor in the tenase complex. By degrading these, APC reduces thrombin generation and limits clot formation. This pathway is a key regulatory mechanism in the coagulation cascade.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect if it lists factor IXa—antithrombin, not APC, primarily inactivates IXa.
**Option B:** Incorrect if it lists factor Xa—APC does not target Xa directly; this is antithrombin’s role.
**Option C:** Incorrect if it lists factor II (thrombin)—APC does

✓ Correct Answer: A. 8a and 5a