Musculoskeletal strain or sprain: Given the acute onset of back pain and bilateral leg weakness, a musculoskeletal issue is a plausible explanation, especially if the patient has recently engaged in strenuous physical activity. The absence of significant laboratory findings (negative ESR, CRP, and MRI) supports this diagnosis, as it suggests there is no underlying inflammatory or structural issue.

Lumbar disc herniation or protrusion: Although the MRI is reported as negative, it's essential to consider that some lumbar disc issues might not be immediately visible on imaging, especially if the herniation is small or if the MRI was not of high quality. Symptoms can include back pain radiating to the legs, which aligns with the patient's presentation.
Infectious or inflammatory myositis: Despite negative ESR and CRP, myositis (inflammation of the muscles) could still be a consideration, especially with fevers present. However, this would typically involve more systemic symptoms and specific muscle groups rather than bilateral leg weakness alone.
Peripheral neuropathy: Conditions affecting the peripheral nerves could lead to weakness and pain. However, the acute onset and presence of fevers might not typically align with common forms of peripheral neuropathy.

Spinal epidural abscess: This is a medical emergency that requires prompt diagnosis and treatment. Despite the negative MRI, if there's a high clinical suspicion (e.g., severe back pain, fever, and neurological deficits), repeating the imaging or considering other diagnostic methods is crucial. The presence of fever and acute back pain, even with a negative initial MRI, warrants careful consideration of this diagnosis.
Cauda equina syndrome: Another emergency, this condition involves compression of the nerve roots in the lower spine, leading to severe back pain, leg weakness, and potentially bladder and bowel dysfunction. Immediate surgical intervention may be necessary, making prompt diagnosis critical.
Spinal cord infarction or ischemia: Although less common, spinal cord infarction can present with acute back pain and bilateral leg weakness. This condition is a neurological emergency requiring immediate attention.

Neurosyphilis: In its tertiary stage, syphilis can cause a wide range of neurological symptoms, including weakness and pain. However, this would be an uncommon presentation and typically associated with other systemic or neurological findings.
Sarcoidosis: This autoimmune disease can affect various parts of the body, including the nervous system, leading to symptoms like weakness and pain. However, it would usually be accompanied by other systemic symptoms or findings suggestive of sarcoidosis.
Vitamin deficiencies (e.g., Vitamin B12 deficiency): Certain vitamin deficiencies can lead to neurological symptoms, including weakness. However, these would typically develop over a longer period and be associated with other signs of deficiency.