**Core Concept**
Lachman's test is a clinical examination used to assess the integrity of the anterior cruciate ligament (ACL) in the knee joint. It involves applying a valgus stress to the knee while it is in a specific degree of flexion, to assess the stability of the joint.

**Why the Correct Answer is Right**
The correct degree of flexion for Lachman's test is between 20-30 degrees. This degree of flexion allows for optimal visualization of the ACL, while also minimizing the effect of other knee stabilizers, such as the posterior cruciate ligament (PCL) and the medial collateral ligament (MCL). When the knee is in this degree of flexion, the ACL is taut, making it easier to assess its integrity.

**Why Each Wrong Option is Incorrect**
**Option A:** 90 degrees - This degree of flexion is too deep and would make it difficult to assess the ACL, as the PCL and MCL would be more taut and interfere with the test.

**Option B:** 0 degrees - This degree of flexion is too straight and would not allow for optimal visualization of the ACL, making it difficult to assess its integrity.

**Option C:** 60 degrees - This degree of flexion is too shallow and would not provide the optimal tension on the ACL, making it difficult to assess its integrity.

**Clinical Pearl / High-Yield Fact**
Remember, Lachman's test should be performed with the knee in 20-30 degrees of flexion, as this allows for optimal visualization of the ACL and minimizes the effect of other knee stabilizers.

**Correct Answer: B. 0 degrees**