**Question:** Increased Prothrombin time results from deficiency of ?

**Core Concept:** Prothrombin time (PT) is a test that measures the overall coagulation process in the blood, reflecting the function of the extrinsic and common pathways of coagulation. It is affected by factors I, II, VII, IX, X, XI, and XII.

**Why the Correct Answer is Right:** The correct answer is D (Factor II deficiency) because Factor II, also known as Prothrombin, plays a crucial role in the extrinsic pathway of coagulation. The extrinsic pathway initiates the coagulation cascade when tissue factor is exposed to blood due to tissue damage. Factor II (Prothrombin) is a key component in this pathway, and its deficiency results in an increased PT.

**Why Each Wrong Option is Incorrect:**

A. Factor I (Vitamin K) deficiency: While Vitamin K is essential for the synthesis of several clotting factors, including Factors II, VII, IX, X, XI, and XII, deficiency of Vitamin K alone would primarily affect these factors, not just Factor II.

B. Factor X deficiency: Factor X is involved in the intrinsic pathway, not the extrinsic pathway. Deficiency of Factor X would result in an increased activated partial thromboplastin time (APTT), not PT.

C. Factor XI deficiency: Factor XI is involved in the intrinsic pathway, not the extrinsic pathway. A deficiency of Factor XI would lead to an increased APTT, not PT.

**Clinical Pearl:** A normal prothrombin time (PT) range varies based on the laboratory, but typically falls between 11-16 seconds for healthy individuals. Increased PT can be indicative of a variety of conditions affecting the coagulation cascade, including deficiencies of coagulation factors and liver disease.

**Correct Answer:** D (Factor II deficiency)