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, but it cannot be ruled out without further examination.
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 can present with pain in the knee or leg and must be considered, especially in patients with risk factors (e.g., recent travel, immobilization). The absence of swelling and the specific nature of the pain make this less likely, but it's crucial not to miss due to its potential severity.
Osteonecrosis: This condition can cause knee pain but typically has a more insidious onset. Given the acute presentation following an injury, it's less likely but should be considered if symptoms persist or worsen over time.

Popliteal Artery Entrapment Syndrome: This rare condition involves compression of the popliteal artery and can cause leg pain, especially with activity. It's more common in young athletes but could be considered in a differential diagnosis for posterior knee pain, especially if other diagnoses are ruled out.
Ganglion Cyst: A ganglion cyst in the posterior knee could cause localized pain and might be considered if a palpable mass is present, although this is not mentioned in the provided history.