In the immediate postoperative period following a primary total knee arthroplasty, a patient suddenly presents with an absolute inability to actively extend the knee against gravity, a palpable soft-tissue defect directly proximal to the superior pole of the patella, and a surprisingly low-riding patella (patella infera) on a lateral radiograph. What is the definitive diagnosis?

Explanation:
The correct option is c. The extensor mechanism is the motor of the knee. An inability to perform a straight leg raise indicates catastrophic failure. The key localizing sign is the palpable gap above the patella, confirming a quadriceps tendon rupture. Furthermore, the radiographic presence of patella infera (or baja) is the classic discriminator: because the quadriceps is no longer pulling the patella superiorly, the intact, strong patellar tendon pulls the isolated patella distally towards the tibial tubercle. Conversely, a patellar tendon rupture would present with a gap below the patella and a high-riding patella (patella alta) pulled proximally by the intact quadriceps. Reference: Insall & Scott Surgery of the Knee.