A Bone marrow transplant recipient patientdeveloped chest infection. On HRCT 'Tree in bud appearance' is seen. Most likely causative agent is:

Pneumocystis [Ref: Harrison I7/e p843; Aicle 'Tree in bud sign' in Journal 'Radiology' and 'Radiographics7 Tree-in-bud is a sign seen in HRCT, most commonly seen with endobronchial spread of Tuberculosis, but can be seen with a wide variety of conditions, most commonly infections. (Described ahead in detail). Theoretically all the options can cause 'Tree-in-bud' sign (although Klebsiella has not been mentioned in the list). So the next clue is Bone marrow transplant. Bone marrow transplant causes a transient state of immunological deficiency leading to a wide variety of oppounistic infections. Among the given options, according to the table and text of Harrison (17/e chapter 'Infections in Transplant Recipients') Pneumocystis and RSV can cause pneumonia after transplant: butPneumocystis is a much more common than RSV. Common Sources of Infections after Hematopoietic Stein Cell Transplantation Period after Transplantation Infection Site Early (<1 Month) Middle (1-4 months) Late (>6 Months) Disseminated Aerobic gram-negative, grain-positive bacteria Nocardia Candida, Aspergillus Encapsulated bacteria (Streptococcus pneumoniae, Haemophilus influenzae, Neisseria meningitidis) Skin and mucous membranes HSV- HHV-6 VZV Lungs Candida, Aspergillus HSV CMV, seasonal respiratory viruses Pneumocystis Toxoplasma Pneumocystis Gastrointestinal tract CMV Kidney BK virus, adenovirus BK virus Brain HHV-6 Toxoplasma Toxoplasma JC virus Bone marrow HHV-6 Tree-in-bud sign The tree-in-bud sign is a finding seen on thin-section computed tomographic images of the lung (HRCT). (Not seen on X-rays) Peripheral, small, centrilobular nodules are connected to linear, branching opacities that have more than one contiguous branching site, thus resembling a budding or, fruiting tree: this is known as tree-in-bud-pattern. It represents dilated and impacted (mucus or pus-filled) centrilobular bronchioles. The presence of tree-in-bud is indicative of small airway disease. It is most commonly associated with endobronchial spread of Mycobacterium tuberculosis. But it can also be seen in a large number of conditions. Pulmonary infectious disorders involving the small airways are the most common causes of the tree-in-bud sign. Any infectious organism, including bacterial, mycobacterial, viral, parasitic, and fungal agents, can involve the small airways and cause a tree-in-bud pattern. Causes of Tree-in-bud appearance Peripheral airway disease Infection Bacterial Mycobacterium tuberculosis M avium-intracellulare complex Staphylococcus aureus Haemophilus influenzae Fungal Aspergillus Pneumocystis carinii, (renamed Pneumocystis jiroveci) Viral Cytomegalovirus Respiratory syncytial virus Congenital disorders Cystic fibrosis Kaagener syndrome Idiopathic disorders Obliterative bronchiolitis Diffuse panbronchiolitis A spiration Inhalation Toxic fumes and gases Immunologic disorders Allergic bronchopulmonary aspergillosis Connective tissue disorders Rheumatoid ahritis Sjo-gren syndrome Peripheral pulmonary vascular disease Neoplasms Primary pulmonary lymphoma Neoplastic pulmonary emboli Gastric cancer Breast cancer Ewing sarcoma Renal cancer