Patellofemoral syndrome presents as pain in the front of the knee, sometimes caused by wearing down, roughening, or softening of the cartilage under the kneecap. Osgood-Schlatter disease is a syndrome of irritation of the patellar ligament at the tibial tuberosity, characterized by painful lumps just below the knee, most often seen in young adolescents. Legg-Calvé-Perthes syndrome is due to disruption of blood flow to the femoral head, leading to avascular necrosis. Congenital scoliosis is a spinal deformity characterized by the curving of the spine at an angle greater than 10 degrees on radiography.

Osgood-Schlatter syndrome of irritation of the patellar ligament at the tibial tuberosity, characterized by painful lumps just below the knee, most often seen in young adolescents. Risk factors may include overzealous exercise but it's ultimately thought to be caused by adolescent bone growth. Osteosarcoma, the most common bone cancer in adolescent populations, can present with pain and a lump or frank swelling. This is caused by actual tumor growth, not by any ligamentous issue. Osteosarcoma does present frequently in the knee, and must be ruled out before a diagnosis of Osgood-Schlatter disease is made. Slipped capital femoral epiphysis causes pain and reduced movement in the hip, rather than the knee. Osteomalacia is a general softening of the bones caused by a deficiency in vitamin D.

Psoriatic arthritis will often show a pencil-in-cup deformity on x-ray. Osteoarthritis will show joint space narrowing, sclerosis, and osteophytes. Rheumatoid arthritis and juvenile rheumatoid arthritis will both show soft tissue swelling, joint space narrowing, and may show marginal erosions. 

Swan neck deformity is often seen in patients with rheumatoid arthritis. This is a state in which the distal interphalangeal joint (DIP) becomes hyperflexed (bends toward the palm) while the proximal interphalangeal joint (PIP) is hyperextended (bending away from the palm). Shepherd's crook deformity is seen in Paget's disease of the bone, pencil-in-cup deformity is seen in psoriatic arthritis, and Heberden nodes are frequently seen in the distal phalanges of patients with osteoarthritis.

Duchenne's is the most common and most severe form of muscular dystrophy, accounting for a full 50% of cases of muscular dystrophy in the US. The next most common type is Becker's, followed by myotonic muscular dystrophy and Emery-Dreifuss.

Lambert-Eaton syndrome is a condition of progressive muscle weakness caused by an auto antibodies to presynaptic calcium channels in the neuromuscular junction. This does not allow calcium to enter the presynaptic cell, which is required for neurotransmitter release into the synaptic cleft. Without the release of neurotransmitter, there will be no muscular response. Myasthenia gravis is an autoimmune condition in which antibodies are formed against acetylcholine receptors, and muscular weakness in multiple sclerosis is caused by destruction of myelin in the CNS due to antibodies against oligodendrocytes. Dermatomyositis is an inflammatory connective tissue condition that may be associated with a post-viral autoimmune reaction or triggered by a paraneoplastic syndrome. 

Histological findings of dermatomyositis will generally reveal mixed B cell and T cell perivascular inflammatory infiltrate and perifascicular muscle fiber atrophy.

Fibrosed muscle fibers and fatty infiltrates are typical of Duchenne's muscular dystrophy, while granulomas in muscle are rare but may be seen in sarcoidosis and other conditions of granulomatous myositis.

A rhabdomyoma is a benign tumors of striated muscle. 90% of adult rhabdomyomas present as round masses in the neck. 

A periorbital heliotrope rash is often seen in dermatomyositis. It is bilateral, symmetric, purpuric, and macular, and erupts over the eyelids and surrounding periorbital area. Malar rash (often called a "butterfly rash") is seen in patients with systemic lupus erythematosus (SLE). Dermatitis herpetiformis is a rash on the elbows and knees generally seen with gluten allergy (celiac disease). Perioral dermatitis is an iatrogenic condition often associated with prolonged use of topical steroid creams. 

Polymyositis is characterized by symmetric proximal muscle weakness in the upper and lower extremities, often without the presence of distal muscle weakness. The first sign is often weakness in the thighs while walking, the inability to rise from a seated position without assistance, or the inability to raise the arms above the head. 

Dermatomyositis generally includes a characteristic periorbital rash. Weakness is expressed in the neck, arms, and legs. Guillain-Barre syndrome is an ascending paralysis: symptoms start in the hands and feet, then spread proximally as it progresses. Muscular weakness in myasthenia gravis is often especially expressed in muscles of the face and head, especially those that control eye and eyelid movement, facial expression, chewing, swallowing, and speaking.