We started with the idea that this is a musculoskeletal problem, supported by the information given that no neurologic defects were detected on physical exam.  The observation of hind limb lameness and the fact that the animal was bearing most weight on the front limbs helped us to localize the problem to the rear legs.  Weight bearing on the front legs also helped us rule out the clicking in the front limbs as an important symptom (though this could be a result of the hind limb problem and ensuing stresses on the front limbs).  In the rear legs, we used the information that there was no cranial drawer sign to eliminate the knee joint as the site of the problem.  Having noted pain on palpation of the hip joint and decreased range of motion in the hip joint, we decided that the hip joint was the location of the musculoskeletal problem.  Further evidence that we think will help us determine the cause of the lameness included: more pain on extension than on flexion, positive Ortalani sign in the left hip (couldn't evaluate in right hip because of greater pain on that side), subluxation of the hip joint, and no joint effusion noted,
	Next we had to decide what kind of musculoskeletal problem we were dealing with.  Two large categories to consider were congenital disorder and acquired disorders.  Certain congenital problems are more common in large breed dogs, such as our patient.  There are also acquired problems for which large breed dogs are at increased risk.  A disorder could be acquired due to injury or due to nutritional factors.  Two types of injury we came up with are those due to trauma and those due to long term stresses/strains.  Because the owner didn't report any traumatic events, we decided that trauma was an unlikely etiology for this lameness.  Injury due to stress/strain is a plausible etiology, because our patient is a large breed dog.  We would like to know his weight and body condition score in order to evaluate whether obesity could be a factor.  A nutrition related disorder is also a likely possibility.  Large breed puppies are at risk of developmental problems resulting from overfeeding, and our patient is fed on a free choice basis.  He is also being fed an adult dog food, which may not be appropriate.  The fact that the dog has a sedentary life style could exacerbate a nutritional problem (as well as disorders due to obesity,).
	Other factors that we think are important are that the lameness is worse in the morning and after exercise, that the dog is uncomfortable on sitting, and that it is reluctant to walk up and down stairs.  Diagnoses we have considered and want to read more about are: hip dysplasia, osteoarthritis secondary to hip dysplasia, osteochondritis dissecans , neoplasia, and a nutritional problem.   At the moment, we think the three most likely causes for the lameness (though we won't rule out the others until we've finished investigating and sharing our learning issues) are joint deformation, joint damage, and a nutritionally based problem.