**Core Concept**
The popliteal fossa is a region at the back of the knee joint, bounded by several anatomical structures. The superior medial border of the popliteal fossa is formed by the tendons of two specific muscles that converge to create this border.

**Why the Correct Answer is Right**
The semitendinosus and semimembranosus muscles are two of the three hamstring muscles that originate from the ischial tuberosity and converge to insert into the tibia and fibula. The tendons of these muscles form the superior medial border of the popliteal fossa, which is a key anatomical landmark for surgeons and clinicians to identify during procedures involving the knee. The confluence of these tendons creates a distinct border that is easily recognizable, especially when viewed from a superior perspective.

**Why Each Wrong Option is Incorrect**
* **Option A:** The tendon of biceps femoris does not form the superior medial border of the popliteal fossa. Instead, it forms the inferior border of the popliteal fossa, along with the popliteus muscle.
* **Option C:** The tendon of plantaris is a small, slender tendon that is often difficult to palpate and is not a significant contributor to the superior medial border of the popliteal fossa.
* **Option D:** The tendinous hiatus of adductor magnus is a small opening in the adductor magnus muscle that allows the hamstring tendons to pass through, but it does not form the superior medial border of the popliteal fossa.

**Clinical Pearl / High-Yield Fact**
When examining the knee, it's essential to recognize the tendons of semitendinosus and semimembranosus as they converge to form the superior medial border of the popliteal fossa. This anatomical landmark can aid in identifying the location of the popliteal artery and vein, which are often constricted in patients with popliteal artery entrapment syndrome.

**✓ Correct Answer: B. Tendons of semitendinosus and semimembranosus**