Cirrhosis with Portal Hypertension: This is the most likely diagnosis given the presence of ascites and thrombocytopenia (platelet count of 49,000). Cirrhosis leads to portal hypertension, which can cause splenic sequestration and subsequent thrombocytopenia. Ascites is also a common complication of cirrhosis due to portal hypertension and hypoalbuminemia.

Splenic Vein Thrombosis: This condition can cause splenic sequestration leading to thrombocytopenia and can also lead to ascites if the thrombosis extends to involve the portal vein or if there is significant splenic enlargement causing portal hypertension.
Nephrotic Syndrome: Although less common, nephrotic syndrome can lead to ascites due to hypoalbuminemia and can also cause thrombocytosis more frequently, but thrombocytopenia can occur, especially if there is significant renal dysfunction affecting platelet production or if there is a concomitant condition causing thrombocytopenia.
Congestive Heart Failure: Right-sided heart failure can lead to ascites and can also cause thrombocytopenia due to splenic congestion, although this is less common.

Abdominal Cancer (e.g., Ovarian, Gastric, Pancreatic): These cancers can cause ascites due to peritoneal carcinomatosis or obstruction of lymphatic or venous drainage. Thrombocytopenia can occur due to marrow infiltration or a paraneoplastic syndrome. Missing these diagnoses can be fatal due to the aggressive nature of these cancers.
Tuberculosis: TB can cause ascites (tuberculous peritonitis) and thrombocytopenia, either due to bone marrow involvement or as part of a systemic inflammatory response. TB is a treatable condition, but missing the diagnosis can lead to significant morbidity and mortality.
Budd-Chiari Syndrome: This condition, caused by hepatic vein thrombosis, can lead to ascites and thrombocytopenia. It is crucial not to miss this diagnosis because it requires specific treatment, such as anticoagulation or thrombolysis, to prevent liver failure and death.

Lymphoma: Both Hodgkin's and non-Hodgkin's lymphoma can cause ascites and thrombocytopenia, either through direct involvement of the peritoneum or through marrow infiltration affecting platelet production.
Amyloidosis: This condition can cause ascites due to gastrointestinal involvement or nephrotic syndrome and can also lead to thrombocytopenia due to marrow involvement or splenic sequestration.
Systemic Lupus Erythematosus (SLE): SLE can cause both ascites and thrombocytopenia as part of its systemic manifestations, including peritonitis and immune-mediated thrombocytopenia.