Considering presenting complain of generalized body swelling diffrentials Like.
Nephrotic syndrome,
Congestive cardiac failure
CKD
Malabsorption,
Chronic liver disease
Cor-pulmonale 
 Hypothyroidism
Lymphedema should be kept in mind.
*As this patient have no history of sob,orthopnea, and PND and jvp is not raised on examination so heart failure is less likely as possibility.
*Patient have no history of jaundice and no stigmata of chronic liver disease on examination so cld is also less likely.
*No history of uremic symptoms like anorexia,nausea ,vomiting,nocturia,&hematuria and normal blood pressure on examination ckd and nephritis is also less likely.
*No history of cough dyspnea hemoptysis and ,cold intolerance constipation,change in voice so corpulmonale & Hypothyroidism is also less likely.
*Patient also have no complains of chronic diarrhea,weight loss,poor dietry intake so malabsorption is also less likely.
*lymphedema will cause painless localised swelling so it is less likely as this patient have generalized swelling.
*Generalized body edema with no remarkable systemic history  and pitting edema upto knees, and normal blood pressure nephrotic seems to be top differential.
About cause of nephrotic syndrome patient should asked for history of recent sore throat,or viral infection,medications history,history of connective tissue disorders(fever, jointpain,rash,mouth ulcers,photosensitivity)& family history of renal diseases.

What tests are required to confirm the diagnosis of Nephrotic Syndrome?

• Urinalysis.(3+ protein in urine re)
• Urine sediment examination(for cells/casts)
• 24 hour urine protein( >3.5g/24 hr)
• Alternative to 24hr urine collection,is spot urine sample for protein/creatnine ratio PCR >300
• Serum albumin <2.5g/dl
• Lipid profile...serum cholestrol may be raised.
• Serological tests : 

Done to identify cause of nephrotic

• ANA levels
• Complement levels
• Hep b/c serology
• Spec
• Measurement of cryoglobulins
• SFLC
• ANCA levels
• RFTs
• Renal ultrasound
• Renal Biopsy​​​​​​​​​​​​​​​​​​​​​

​​​​​​​A renal biopsy is not indicated in adults with nephrotic syndrome from an obvious cause.for example longstanding diabetes​​​​​​​​​​​​​​performed in cases with.

• Idiopathic nephrotic 
• Persistent proteinuria despite 4 weeks of steriod therapy.
• Frequent relapses or steriod dependency or steriod resistance cases.
• Significant nephritic menifestations.
• Persistent or gross hematuria
• Falling complement levels
• Development of renal insufficency.
• Sustained hypertension unrelated to steriod therapy