40 year old lady presented with sudden onset of breathing difficulty. Clinical examination was essentially normal except for tachycardia and tachypnea. She gives history of two episodes of lower limb pain and swelling, first episode was 15 years back during her first pregnancy and second episode 3 months back. What would be the hematological abnormality respoinsible for her condition:

**Core Concept**
The patient's presentation of recurrent lower limb pain and swelling, coupled with sudden onset of breathing difficulty, suggests a diagnosis of a hematological disorder that predisposes her to thromboembolic events. The most likely condition is a hypercoagulable state, characterized by an increased tendency for blood clot formation.

**Why the Correct Answer is Right**
The patient's symptoms are suggestive of a condition known as **Antithrombin III deficiency**, a congenital disorder where the levels of antithrombin III, a protein that inhibits thrombin and other clotting factors, are low. This deficiency leads to an increased risk of thrombosis, including deep vein thrombosis (DVT) and pulmonary embolism (PE), which explains the patient's sudden onset of breathing difficulty. The recurrent episodes of lower limb pain and swelling are likely due to previous DVT episodes.

**Why Each Wrong Option is Incorrect**
* **Option A:** **Factor V Leiden deficiency** is a different congenital disorder that leads to a hypercoagulable state, but it is not the most likely diagnosis in this patient, given her history of recurrent episodes of lower limb pain and swelling.
* **Option B:** **Protein C deficiency** is another genetic disorder that predisposes individuals to thrombosis, but it is less common than Antithrombin III deficiency.
* **Option C:** **Disseminated Intravascular Coagulation (DIC)** is a acquired condition characterized by both thrombosis and bleeding, but it is not a congenital disorder and is not the most likely diagnosis in this patient.

**Clinical Pearl / High-Yield Fact**
Recurrent episodes of lower limb pain and swelling, particularly in a woman with a history of pregnancy, should raise suspicion for a hypercoagulable state, such as Antithrombin III deficiency.

**Correct Answer:** C.

✓ Correct Answer: B. Resistance to activated protein C