**Core Concept**
The lift-off test assesses the function of the subscapularis muscle, which is one of the four rotator cuff muscles responsible for internal rotation of the shoulder. This test evaluates muscle strength and integrity by detecting weakness or absence of internal rotation.

**Why the Correct Answer is Right**
The lift-off test is performed by asking the patient to internally rotate the shoulder while the examiner holds the arm and then releases it. The patient must be able to maintain the position without dropping the arm. The subscapularis muscle is the primary internal rotator of the shoulder, and its weakness or dysfunction is detected when the arm "drops" during the test. This test is highly specific for subscapularis pathology, especially in cases of rotator cuff injury.

**Why Each Wrong Option is Incorrect**
Option A: Supraspinatus is responsible for abduction of the shoulder and is assessed by the Neer or Hawkins-Kennedy tests, not the lift-off test.
Option C: Infraspinatus performs external rotation and is evaluated via the external rotation test (e.g., O'Brien’s test), not lift-off.
Option D: Biceps brachii contributes to flexion and supination but is not involved in internal rotation; its role is assessed via the biceps tendon test or resisted flexion.

**Clinical Pearl / High-Yield Fact**
Always remember: The lift-off test is specific for subscapularis weakness. A negative test indicates normal internal rotation strength, while a positive test (arm drops) suggests subscapularis dysfunction — a common finding in rotator cuff tears.

✓ Correct Answer: B. Subscapularis