Arrange the following in the sequential order of appearance during intrinsic clotting pathway: A. Phospholipid B. Prekallikrien C. Activated factor V D. Factor XIII
**Core Concept**
The intrinsic clotting pathway is a series of biochemical reactions that lead to the formation of a blood clot. This pathway is activated when blood comes into contact with a foreign surface, such as a broken blood vessel. The intrinsic pathway involves a series of enzyme activations and substrate conversions that ultimately lead to the formation of thrombin and fibrin.
**Why the Correct Answer is Right**
The intrinsic clotting pathway begins with the activation of factor XII (Hageman factor), which is not listed in the options. However, the correct sequence of events can be inferred from the given options. The activation of factor XII leads to the activation of factor XI, which in turn activates factor IX. Factor IX is then activated to factor IXa, which activates factor X to factor Xa. Factor Xa, in combination with factor Va, activates prothrombin to thrombin. Thrombin then converts fibrinogen to fibrin, which forms the blood clot. Factor XIII, also known as fibrin stabilizing factor, is then activated to cross-link the fibrin molecules, stabilizing the clot.
**Why Each Wrong Option is Incorrect**
* **Option A:** Phospholipid is not a sequential step in the intrinsic clotting pathway. However, phospholipid surfaces are required for the assembly of the intrinsic clotting complex. Phospholipids are essential for the binding of factors V and X to the platelet membrane.
* **Option B:** Prekallikrein is involved in the contact activation system, which is part of the intrinsic clotting pathway. However, it is not a sequential step in the intrinsic clotting pathway. Prekallikrein is activated to kallikrein, which in turn activates factor XII.
* **Option C:** Activated factor V is part of the intrinsic clotting pathway, but it is not the correct sequential step. Factor V is activated to factor Va, which is a component of the prothrombinase complex that activates prothrombin to thrombin.
**Clinical Pearl / High-Yield Fact**
The intrinsic clotting pathway is often remembered using the mnemonic "KISS": Keep It Simple, Stupid. The correct sequence of events in the intrinsic clotting pathway is: factor XII, factor XI, factor IX, factor X, factor Va, and factor XIII.
**Correct Answer:** D. Factor XIII is the correct sequential step in the intrinsic clotting pathway, as it is the last step in the formation of a stable blood clot.