Posterior Knee Soft Tissue Injury (e.g., posterior knee ligament sprain or strain): This diagnosis is most likely due to the mechanism of injury (stepping down from a bus with the left foot going backward), the location of the pain (posterior knee), and the nature of the pain (electrifying shock-like pain that occurs with knee flexion and standing up from a bent position). The normal X-ray findings also support a soft tissue injury rather than a bony injury.

Meniscal Tear: The pain with knee flexion and the inability to run due to pain could suggest a meniscal tear. However, meniscal tears often present with locking or catching sensations, which are not mentioned in the history.
Hamstring Strain: Given the posterior knee pain and the mechanism of injury, a hamstring strain is possible. This would typically present with pain in the posterior thigh rather than localized to the posterior knee.
Biceps Femoris Tendinopathy: As part of the hamstring group, biceps femoris tendinopathy could cause posterior knee pain, especially with activities involving knee flexion.

Deep Vein Thrombosis (DVT): Although less likely given the acute onset of pain following a specific injury, DVT should be considered, especially if there are risk factors such as recent travel (e.g., driving long distances) or immobilization. The pain description and lack of swelling do not strongly suggest DVT, but it's a critical diagnosis not to miss due to its potential severity.
Osteonecrosis: This could be a consideration if the pain persists and does not respond to treatment, especially in the absence of significant trauma. However, osteonecrosis would be less likely given the acute onset following a specific injury.

Popliteal Artery Entrapment Syndrome: This rare condition involves compression of the popliteal artery and could present with claudication-like symptoms, including pain in the posterior knee and calf with activity. However, it would be unusual for this condition to present acutely following a minor injury.
Ganglion Cyst: A ganglion cyst in the posterior knee could cause localized pain, but this would typically be associated with a palpable mass, which is not mentioned in the objective findings.
Neurogenic Causes (e.g., S1 Radiculopathy): While less likely given the specific mechanism of injury and the localized nature of the pain, neurogenic causes could present with radiating pain. However, these would typically be associated with other neurological symptoms such as numbness, tingling, or weakness, which are not described.