What is the diagnosis for a patient presenting with a constant dull ache on the right side of the lower back, which began 2 weeks ago after a motor vehicle accident (MVA), with pain ranging from 3/10 to 6/10, and no associated symptoms such as numbness, tingling, bowel, or bladder incontinence?

Musculoskeletal Strain: Given the patient's history of being involved in a motor vehicle accident (MVA) 2 weeks prior, where their car was stopped and hit from behind, a musculoskeletal strain is the most likely diagnosis. The constant dull ache in the right side lower back, exacerbated by activity, and the absence of neurological symptoms such as numbness, tingling, or bowel/bladder incontinence, support this diagnosis. The pain scale of 3/10 to 6/10 also aligns with the expected pain level for a musculoskeletal injury.

Lumbar Disc Herniation: Although the patient denies radiation of pain, numbness, or tingling, a lumbar disc herniation could still be considered, especially given the mechanism of injury. The force from the MVA could have caused a disc herniation, which might not always present with typical radicular symptoms initially.
Ligamentous Sprain: Similar to musculoskeletal strain, ligamentous sprains are common in MVAs due to the sudden force applied to the body. This could cause pain and stiffness in the lower back, consistent with the patient's symptoms.
Sacroiliac Joint Dysfunction: The sacroiliac joint can be injured in MVAs, leading to pain in the lower back that may radiate to the buttock or thigh. Although the patient denies radiation, sacroiliac joint dysfunction should be considered due to the mechanism of injury.

Abdominal Aortic Aneurysm (AAA): Although less likely given the patient's age is not specified and the absence of abdominal pain, an AAA could present with back pain and is a potentially life-threatening condition if ruptured. The mechanism of injury could theoretically contribute to an AAA, especially if the patient has pre-existing vascular disease.
Spinal Fracture: A spinal fracture, including a vertebral compression fracture, could occur from the MVA, especially if the patient was not properly restrained or if the collision was at a high speed. This is a critical diagnosis not to miss due to the potential for neurological compromise.
Kidney Injury: Direct trauma to the kidneys or a renal contusion could result from the MVA, presenting with flank or back pain. Hematuria might not always be present, making this a diagnosis that requires consideration.

Osteoporotic Compression Fracture: If the patient has underlying osteoporosis, the force from the MVA could have caused a vertebral compression fracture, leading to back pain. This would be less likely in a younger patient without known osteoporosis.
Spinal Infection: Although rare, a spinal infection (e.g., discitis or osteomyelitis) could present with back pain and might be considered if the patient has risk factors such as immunocompromised status or recent spinal procedure. However, this would typically be accompanied by other symptoms like fever or elevated inflammatory markers.