Trochanteric bursitis: This condition is a common cause of hip pain, especially in older adults. The sudden onset of pain while walking downhill on uneven ground could irritate the trochanteric bursa, leading to inflammation and pain. The location and nature of the pain, along with the patient's age and activity, make this a plausible diagnosis.

Gluteus medius tendonitis: Similar to trochanteric bursitis, tendonitis of the gluteus medius tendon can cause hip pain, especially with activities that involve uneven ground or downhill walking, which can strain the tendon.
Ligament sprain or strain: The uneven ground could lead to an abnormal gait or sudden twisting, resulting in a sprain or strain of the ligaments around the hip.
Osteoarthritis: Although osteoarthritis is a chronic condition, an acute exacerbation could be triggered by walking on uneven ground, leading to sudden onset of pain.

Hip fracture: While less likely, a hip fracture (e.g., femoral neck fracture) is a critical diagnosis that must not be missed. It can occur in older adults, especially those with osteoporosis, after a fall or even a sudden twist on uneven ground.
Avascular necrosis of the femoral head: This condition can cause sudden hip pain and is more common in older adults. It's crucial to diagnose early to prevent further damage.
Septic arthritis: Although rare, septic arthritis is a medical emergency that requires prompt diagnosis and treatment. It can present with sudden onset of hip pain and is more likely in older adults or those with compromised immune systems.

Stress fracture: A stress fracture of the femur or pelvis could cause hip pain, especially with activities involving uneven ground. However, this is less common in the absence of repetitive stress or overuse.
Piriformis syndrome: This condition involves irritation of the sciatic nerve by the piriformis muscle and can cause hip and buttock pain. It's less common but should be considered if other diagnoses are ruled out.
Intra-abdominal or pelvic pathology: Rarely, hip pain can be referred from intra-abdominal or pelvic conditions such as a hernia, appendicitis, or ovarian torsion. These conditions are less likely but must be considered in the differential diagnosis to avoid missing a critical diagnosis.